**Measuring the Isotopic Composition of Solar Wind Noble Gases**

Alex Meshik, Charles Hohenberg, Olga Pravdivtseva and Donald Burnett *Washington University, Saint Louis, MO California Institute of Technology, Pasadena, CA USA* 

## **1. Introduction**

92 Exploring the Solar Wind

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It is generally accepted that the primitive Sun, which contains the vast majority of the mass of the solar system, has the same composition as the primitive solar nebula, and that the contemporary Sun has a similar composition except perhaps for light elements modified in main sequence hydrogen burning. The diversity of isotopic and elemental compositions now observed in various solar system reservoirs is most likely the result of subsequent modification and noble gases can provide us with valuable tools to understand the evolutionary paths leading to these different compositions. However, to do this we need to know the composition of the Sun with sufficient precision to delineate the different paths and processes leading to the variations observed and how the present solar wind noble gases may differ from that composition.

Solar optical spectroscopy, the main source of early knowledge about composition of the Sun, does not reveal isotopic information and noble gases do not have useful lines in the solar spectra except for He which was, interestingly, first found in the Sun by this method. Early estimations of solar abundances were based on the combination of photospheric spectral data and laboratory analysis of primitive meteorites which carry a clear signature of their original noble gases. This approach is justified by the fact that the CI chondrites, a rare class of primitive meteorites, and photospheric spectroscopy yield almost identical abundances of most nonvolatile elements. Since meteorites, which were formed by the preferential accretion of solids, clearly differ from the unfractionated solar nebula, the composition of primitive meteorites does not provide a suitable measure of solar system volatiles. Noble gases in meteorites are depleted by many orders of magnitude compared with the solar nebula and, although lunar soils and breccias, implanted with solar wind noble gases, did provide a needed ground truth, neither by themselves could provide a good values for solar volatiles. The first "best estimate" of solar abundances was found by interpolating between adjacent non-volatile elements (Anders & Grevesse, 1989), supplemented with the lunar data, and the later updates (Palme and Beer 1993, Grevesse & Sauval 1998, Lodders, 2010) provided presumably more reliable estimates, but all failed to supply precise isotopic, or even elemental, compositions of the solar noble gases.

Measuring the Isotopic Composition of Solar Wind Noble Gases 95

flux was insufficient to produce the quantity of SEP observed. The proposed SEP component was assumed to have more energy than the typical SW of ~ 1 keV/nucleon but far less energy than the solar flares (~1 MeV/nucleon), commonly referred to as SEP particles by the

There were still more problems in interpreting noble gases released from lunar soils, lunar breccias, and gas-rich meteorites even though they were dominated by the solar wind. Radiation damage and disruption caused by solar wind hydrogen in surfaces exposed to the SW for as little as tens of years leads to lattice defects, enhanced diffusive losses and accelerated surface erosion (compared with laboratory simulations on undamaged samples). With enhanced diffusive losses comes an exaggerated isotopic fractionation effect. The Apollo Solar Wind Composition (SWC) experiment was designed to measure the light solar wind noble gases on pristine metallic surfaces without such effects, and it was successfully carried out during Apollo 11-16, the first five lunar landing missions. In this experiment the Apollo crews exposed Al- and Pt-foils to the SW for up to 45 hours and the foils were returned to Earth where the directly implanted SW-He, Ne and Ar were analyzed in noble gas mass spectrometers (Geiss et al., 1972, 2004). Since the exposure was short enough to avoid saturation effects, it was too short for analyses of the least abundant Xe and Kr and even the Ar analysis (36Ar/38Ar = 5.4 ± 0.3) was not sufficiently precise to delineate solar from terrestrial argon. Moreover, the light noble gases implanted in the foils were easily contaminated by small amounts of dust from the lunar regolith which contained large concentrations of noble gases implanted in the dust by the solar wind but altered by the processes just discussed. Although the dust was largely removed, and these foils represented the best solar wind data for the light noble gases at that time, the problem was

The Genesis Space Mission (http://genesismission.jpl.nasa.gov/; Burnett et al, 2003) provided a dramatic improvement over the SWC experiment. With more than 400-times longer exposure, much purer collector materials, and free from contamination by most other components, it collected pure contemporary solar wind from outside of the terrestrial magnetosphere. Not only could the compositions of noble gases be determined with new precision, but many other elements could be measured as well including N and O. Ultrapure materials, prepared exclusively for the purpose of SW collection with low-blank analyses (Jurewicz at al., 2003), were exposed to the SW for 27 months at the L1 Lagrangian point, a pseudo-stable location which orbits with the Earth between the Sun and the Earth. On September 8, 2004 the Genesis returned capsule landed although, due to a parachute failure (Genesis Mishap Investigation Report, 2005), it was not as "soft" as was originally planned. The "hard" landing caused significant delay in SW analyses because of the need to identify and clean several thousand fragments of broken collectors and, in many incidences, develop new techniques for the analyses. In spite of this, Genesis turned out to be a very successful mission with most of the original objectives met, in fact it was the first successful sample return mission since the Apollo era. We present here the results of a comprehensive analysis of SW noble gases collected by the Genesis SW-collectors. All of the analyses reported here were performed at Washington University in St. Louis using massspectrometers especially developed for Genesis and laser extraction techniques that continued to evolve during the course of analyses for the mission, descriptions of which are

space physics community, so this terminology was confusing.

not completely resolved.

presented in this study.

Light solar wind noble gases were directly measured by mass spectrometers on various spacecrafts. The most recent of those missions were WIND, Ulysses, SOHO (Solar and Heliospheric Observatory and ACE (Advanced Composition Explorer), see NASA website and review papers (i.e. Wimmer-Schweingruber, 1999, 2001). But the flux is low for the heavier noble gases, and the compositions of the light gases are known to vary with energy, so none of these provided solar isotopic and elemental abundances with sufficient precision.

The Apollo and Luna missions delivered samples of solar wind (SW) accumulated over million years in the lunar regolith. Besides the shallowly implanted SW noble gases, these samples also contained deeper noble gases mainly produced by the spallation reactions of cosmic ray protons and secondary particles, and compositions may be modified by diffusion. In order to delineate the various components, these gases were extracted using stepped pyrolysis and analyzed in sensitive mass spectrometers operated in the static mode. At low temperatures the released gases were dominated by "surface-correlated", mostly SW, while at high temperatures they were mainly "volume-correlated", mostly spallationproduced, noble gases and other in-situ contributions such as radioactive decay. To determine SW compositions, isotope correlation analyses were used. In three-isotope correlation plots two component mixtures are distributed in linear arrays, while for three component mixtures the data fill two-dimensional figures whose apexes define the pure end-member compositions. Several independent analyses using slightly different databases, slightly different techniques and somewhat different assumptions yielded several slightly different compositions for the heavy solar noble gases Xe and Kr. These compositions are referred to as BEOC 10084 (Eberhardt et al., 1970), SUCOR (Podosek et al., 1971) and BEOC 12001 (Eberhardt et al., 1972). Later, in attempts to better separate SW and gases that resided more deeply, stepped extractions from grain-size separates of the lunar fines were carried out (Drozd et al, 1972; Behrmann, et al, 1973; Basford et al., 1973; Bernatowicz et al., 1979) and a lunar soil 71501 was studied using in-vacuo stepped-etching technique (Wieler & Baur., 1995). This technique called CSSE (for Closed System Stepped Etching) allowed a better depth resolution of SW noble gases while reducing the potential mass-fractionation during stepped pyrolysis. Beside lunar soils (Wieler et al., 1986), SW-rich meteorites (e.g. Pesyanoe) were studied and, after significant spallation and other corrections, these studies yielded yet another composition for heavy noble gases in the solar wind (Pepin et al., 1995). All of these determinations of SW noble gases were in general agreement but there were slight differences in composition and no general consensus as to which was best.

One of the major complications was the presence of two seemingly distinct SW noble gas components, apparently residing at different depths within a given target: the "normal" SW and the more deeply implanted, presumably the more energetic component, subsequently labeled SEP, for the Solar Energetic Particle component (not to be confused with SEP, a label for solar flares by the solar physics community). The SEP "component" was first identified by Black & Pepin (1969) and Black (1972) with an apparent 20Ne/22Ne ratio <11, much smaller than "normal" SW value of 20Ne/22Ne = 13.7 ± 0.3, and they then called it Ne-C or Ne-SF assuming that it was produced by Solar Flares. The low 20Ne/22Ne ratio was supported by direct Ne analyses in solar flares (Dietrich & Simpson, 1979, Mewaldt et al., 1981, 1984), so this interpretation gained even more supporters (i.e. Nautiyal et al., 1981, 1986, Benkert at al., 1993) but it had to be much lower in energy than these solar flares. Wieler et al (1986) suggested replacing the term SF with SEP (for Solar Energetic Particles) since the SEP must have energies intermediate between SW and SF ions, and because the SF

Light solar wind noble gases were directly measured by mass spectrometers on various spacecrafts. The most recent of those missions were WIND, Ulysses, SOHO (Solar and Heliospheric Observatory and ACE (Advanced Composition Explorer), see NASA website and review papers (i.e. Wimmer-Schweingruber, 1999, 2001). But the flux is low for the heavier noble gases, and the compositions of the light gases are known to vary with energy, so none of these provided solar isotopic and elemental abundances with sufficient precision. The Apollo and Luna missions delivered samples of solar wind (SW) accumulated over million years in the lunar regolith. Besides the shallowly implanted SW noble gases, these samples also contained deeper noble gases mainly produced by the spallation reactions of cosmic ray protons and secondary particles, and compositions may be modified by diffusion. In order to delineate the various components, these gases were extracted using stepped pyrolysis and analyzed in sensitive mass spectrometers operated in the static mode. At low temperatures the released gases were dominated by "surface-correlated", mostly SW, while at high temperatures they were mainly "volume-correlated", mostly spallationproduced, noble gases and other in-situ contributions such as radioactive decay. To determine SW compositions, isotope correlation analyses were used. In three-isotope correlation plots two component mixtures are distributed in linear arrays, while for three component mixtures the data fill two-dimensional figures whose apexes define the pure end-member compositions. Several independent analyses using slightly different databases, slightly different techniques and somewhat different assumptions yielded several slightly different compositions for the heavy solar noble gases Xe and Kr. These compositions are referred to as BEOC 10084 (Eberhardt et al., 1970), SUCOR (Podosek et al., 1971) and BEOC 12001 (Eberhardt et al., 1972). Later, in attempts to better separate SW and gases that resided more deeply, stepped extractions from grain-size separates of the lunar fines were carried out (Drozd et al, 1972; Behrmann, et al, 1973; Basford et al., 1973; Bernatowicz et al., 1979) and a lunar soil 71501 was studied using in-vacuo stepped-etching technique (Wieler & Baur., 1995). This technique called CSSE (for Closed System Stepped Etching) allowed a better depth resolution of SW noble gases while reducing the potential mass-fractionation during stepped pyrolysis. Beside lunar soils (Wieler et al., 1986), SW-rich meteorites (e.g. Pesyanoe) were studied and, after significant spallation and other corrections, these studies yielded yet another composition for heavy noble gases in the solar wind (Pepin et al., 1995). All of these determinations of SW noble gases were in general agreement but there were

slight differences in composition and no general consensus as to which was best.

One of the major complications was the presence of two seemingly distinct SW noble gas components, apparently residing at different depths within a given target: the "normal" SW and the more deeply implanted, presumably the more energetic component, subsequently labeled SEP, for the Solar Energetic Particle component (not to be confused with SEP, a label for solar flares by the solar physics community). The SEP "component" was first identified by Black & Pepin (1969) and Black (1972) with an apparent 20Ne/22Ne ratio <11, much smaller than "normal" SW value of 20Ne/22Ne = 13.7 ± 0.3, and they then called it Ne-C or Ne-SF assuming that it was produced by Solar Flares. The low 20Ne/22Ne ratio was supported by direct Ne analyses in solar flares (Dietrich & Simpson, 1979, Mewaldt et al., 1981, 1984), so this interpretation gained even more supporters (i.e. Nautiyal et al., 1981, 1986, Benkert at al., 1993) but it had to be much lower in energy than these solar flares. Wieler et al (1986) suggested replacing the term SF with SEP (for Solar Energetic Particles) since the SEP must have energies intermediate between SW and SF ions, and because the SF flux was insufficient to produce the quantity of SEP observed. The proposed SEP component was assumed to have more energy than the typical SW of ~ 1 keV/nucleon but far less energy than the solar flares (~1 MeV/nucleon), commonly referred to as SEP particles by the space physics community, so this terminology was confusing.

There were still more problems in interpreting noble gases released from lunar soils, lunar breccias, and gas-rich meteorites even though they were dominated by the solar wind. Radiation damage and disruption caused by solar wind hydrogen in surfaces exposed to the SW for as little as tens of years leads to lattice defects, enhanced diffusive losses and accelerated surface erosion (compared with laboratory simulations on undamaged samples). With enhanced diffusive losses comes an exaggerated isotopic fractionation effect. The Apollo Solar Wind Composition (SWC) experiment was designed to measure the light solar wind noble gases on pristine metallic surfaces without such effects, and it was successfully carried out during Apollo 11-16, the first five lunar landing missions. In this experiment the Apollo crews exposed Al- and Pt-foils to the SW for up to 45 hours and the foils were returned to Earth where the directly implanted SW-He, Ne and Ar were analyzed in noble gas mass spectrometers (Geiss et al., 1972, 2004). Since the exposure was short enough to avoid saturation effects, it was too short for analyses of the least abundant Xe and Kr and even the Ar analysis (36Ar/38Ar = 5.4 ± 0.3) was not sufficiently precise to delineate solar from terrestrial argon. Moreover, the light noble gases implanted in the foils were easily contaminated by small amounts of dust from the lunar regolith which contained large concentrations of noble gases implanted in the dust by the solar wind but altered by the processes just discussed. Although the dust was largely removed, and these foils represented the best solar wind data for the light noble gases at that time, the problem was not completely resolved.

The Genesis Space Mission (http://genesismission.jpl.nasa.gov/; Burnett et al, 2003) provided a dramatic improvement over the SWC experiment. With more than 400-times longer exposure, much purer collector materials, and free from contamination by most other components, it collected pure contemporary solar wind from outside of the terrestrial magnetosphere. Not only could the compositions of noble gases be determined with new precision, but many other elements could be measured as well including N and O. Ultrapure materials, prepared exclusively for the purpose of SW collection with low-blank analyses (Jurewicz at al., 2003), were exposed to the SW for 27 months at the L1 Lagrangian point, a pseudo-stable location which orbits with the Earth between the Sun and the Earth.

On September 8, 2004 the Genesis returned capsule landed although, due to a parachute failure (Genesis Mishap Investigation Report, 2005), it was not as "soft" as was originally planned. The "hard" landing caused significant delay in SW analyses because of the need to identify and clean several thousand fragments of broken collectors and, in many incidences, develop new techniques for the analyses. In spite of this, Genesis turned out to be a very successful mission with most of the original objectives met, in fact it was the first successful sample return mission since the Apollo era. We present here the results of a comprehensive analysis of SW noble gases collected by the Genesis SW-collectors. All of the analyses reported here were performed at Washington University in St. Louis using massspectrometers especially developed for Genesis and laser extraction techniques that continued to evolve during the course of analyses for the mission, descriptions of which are presented in this study.

Measuring the Isotopic Composition of Solar Wind Noble Gases 97

or withstand spacecraft mechanical stress, Genesis collectors are made using an active layer of collector material ~ 1 µm thick deposited on sapphire substrate material (Jurewicz at al., 2003). These "sandwiches" turned out to be very convenient for laser extraction techniques because the laser energy is confined to the coatings where all implanted SW reside. Thus, there is no thermal coupling to the substrate so the substrate does not contribute to background effects from either indigenous or trapped atmospheric noble gases, both of which are ubiquitously present at various levels in all materials including the substrate material used here. This was another significant improvement of the Genesis SW collectors over those of the Apollo SWC experiment in which the foils were self-supporting aluminum films ~ 15 µm thick with the noble gases extracted by pyrolysis (complete melting) of the foils, resulting in noble gas backgrounds at least 15 times higher than in the Genesis 1-µm ultrapure coatings on sapphire.

Two types of SW-collectors were used in this study: AloS (~ 15 µm Aluminum deposited on Sapphire substrates) and PAC (Polished Aluminum Collectors). The latter consisted of 0.05" thick highly polished T6 6061 Al-alloy, a material not intended to function as a SW collector but as a thermal control surface. After the hard landing the PAC turned out to be the largest area available for the analyses of SW noble gases, especially important for the heavy noble gases.

Prior to Genesis mission, we had developed a unique method for collecting low-energy cometary volatiles by growing low-Z metal films on sapphire substrates (Hohenberg at al., 1997). This method utilized a technique we referred to as "active capture" and involved the "anomalous adsorption" of Xe and Kr at chemically active sites, permanently entrapping them in the growing metal films (Hohenberg et al., 2002). Anomalous adsorption is a term we use to distinguish the chemical bonding of heavy noble gases from conventional Van der Waals adsorption and requires the availability of unfilled bonds. In the course of refining the active capture technique, low background laser ablation methods were developed to extract noble gases from these films, during which backgrounds, trapping efficiencies and other properties of these films were extensively studied. A natural extension of this work led to the optimized Genesis SW collectors and recovery techniques of impinged SW gases.

The aluminum on Sapphire (AloS) collectors have many advantages over other thin films and over the polished aluminum collectors (PAC). First, Al has a relatively low melting point compared to other metallic films, requiring less laser power for ablation and therefore less energy deposited in the laser extraction cell which results in lower noble gas backgrounds (blanks), especially important for the low abundance heavy noble gases. Second, the low-Z of the target aluminum means that the backscatter of SW ions will be much smaller compared with other potential collectors such as Au, requiring a much smaller back scatter correction especially for the light noble gases where the projectile Z is also low. Third, aluminum is a good conductor, eliminating any charging effects. Finally, the rapid diffusion of hydrogen in Al (compared with Si and other collector materials) reduces lattice damage and lattice distortion effects caused by the huge amounts of SW hydrogen which can adversely affect the quantitative retention of the light noble gases. Moreover, these SW hydrogen effects are difficult to properly model or simulate so reducing the

The main disadvantage of AloS is that the Al coating is somewhat fragile and can be easily damaged. Several scratched fragments of AloS have demonstrated measurable SW-He

problem is the best way to minimize the effect (Meshik et al., 2000).

**2.1 Aluminum solar wind collectors** 

## **2. Solar wind collection**

Highly ionized solar wind ions, ~ 1keV/nucleon energy, can be effectively collected by most solids, penetrating the lattice, losing energy by scattering, and coming to rest at a depth (range) that is characteristic of the ion, its energy and the target material. Once an energetic ion penetrates a solid it becomes quickly stripped of all residual electrons. After it slows down sufficiently to pick up electrons from the lattice, its charge state is determined by its instantaneous energy and the composition of the target material. Each scattering outcome depends upon specific impact parameters and the interactions between the incident ions and the lattice electrons result in quantum exclusions of some otherwise available states. The constantly changing energy makes these effects even more complex so, in spite of long efforts of many renowned physicists (e.g. Fermi, 1940; Bohr, 1940; Knipp and Teller, 1941 and others), no analytical solution for the ranges of ions has been found. Instead, a Monte Carlo approach is commonly used to simulate each scattering, statistically tracking the trajectory and energy of a population of energetic ions penetrating solid materials and arriving at a distribution of expected ranges as a function of the ion, the initial energy and the target material.

SRIM (the **S**topping and **R**ange of **I**ons in **M**atter) is a suite of the computer codes which calculate the ranges of ions from 10 eV/nucleon to 2 GeV/nucleon in various materials based upon Monte Carlo simulations of successive scatterings, with intermediate trajectories and energies defining conditions for subsequent scatterings, leading to a final distribution of penetration depths (Ziegler et al., 2008, and available at www.srim.org). Figure 1 displays SRIM-2008 results for normal incident solar wind noble gases stopping in aluminum. As can be seen, while heavy noble gases penetrate deeper than lighter ones, all implanted SW-noble gases reside within the outermost 0.3 µm. Therefore, the active portion of the Genesis SWcollectors need not be thicker than a micron or so. Since such a thin foil cannot support itself

Fig. 1. Depth profiles of noble gases implanted into aluminum at 1kV/nucleon energy. Calculated by SRIM-2008 (Ziegler et al., 2008).

or withstand spacecraft mechanical stress, Genesis collectors are made using an active layer of collector material ~ 1 µm thick deposited on sapphire substrate material (Jurewicz at al., 2003). These "sandwiches" turned out to be very convenient for laser extraction techniques because the laser energy is confined to the coatings where all implanted SW reside. Thus, there is no thermal coupling to the substrate so the substrate does not contribute to background effects from either indigenous or trapped atmospheric noble gases, both of which are ubiquitously present at various levels in all materials including the substrate material used here. This was another significant improvement of the Genesis SW collectors over those of the Apollo SWC experiment in which the foils were self-supporting aluminum films ~ 15 µm thick with the noble gases extracted by pyrolysis (complete melting) of the foils, resulting in noble gas backgrounds at least 15 times higher than in the Genesis 1-µm ultrapure coatings on sapphire.

#### **2.1 Aluminum solar wind collectors**

96 Exploring the Solar Wind

Highly ionized solar wind ions, ~ 1keV/nucleon energy, can be effectively collected by most solids, penetrating the lattice, losing energy by scattering, and coming to rest at a depth (range) that is characteristic of the ion, its energy and the target material. Once an energetic ion penetrates a solid it becomes quickly stripped of all residual electrons. After it slows down sufficiently to pick up electrons from the lattice, its charge state is determined by its instantaneous energy and the composition of the target material. Each scattering outcome depends upon specific impact parameters and the interactions between the incident ions and the lattice electrons result in quantum exclusions of some otherwise available states. The constantly changing energy makes these effects even more complex so, in spite of long efforts of many renowned physicists (e.g. Fermi, 1940; Bohr, 1940; Knipp and Teller, 1941 and others), no analytical solution for the ranges of ions has been found. Instead, a Monte Carlo approach is commonly used to simulate each scattering, statistically tracking the trajectory and energy of a population of energetic ions penetrating solid materials and arriving at a distribution of expected ranges as a function of the ion, the initial energy and

SRIM (the **S**topping and **R**ange of **I**ons in **M**atter) is a suite of the computer codes which calculate the ranges of ions from 10 eV/nucleon to 2 GeV/nucleon in various materials based upon Monte Carlo simulations of successive scatterings, with intermediate trajectories and energies defining conditions for subsequent scatterings, leading to a final distribution of penetration depths (Ziegler et al., 2008, and available at www.srim.org). Figure 1 displays SRIM-2008 results for normal incident solar wind noble gases stopping in aluminum. As can be seen, while heavy noble gases penetrate deeper than lighter ones, all implanted SW-noble gases reside within the outermost 0.3 µm. Therefore, the active portion of the Genesis SWcollectors need not be thicker than a micron or so. Since such a thin foil cannot support itself

Fig. 1. Depth profiles of noble gases implanted into aluminum at 1kV/nucleon energy.

Calculated by SRIM-2008 (Ziegler et al., 2008).

**2. Solar wind collection** 

the target material.

Two types of SW-collectors were used in this study: AloS (~ 15 µm Aluminum deposited on Sapphire substrates) and PAC (Polished Aluminum Collectors). The latter consisted of 0.05" thick highly polished T6 6061 Al-alloy, a material not intended to function as a SW collector but as a thermal control surface. After the hard landing the PAC turned out to be the largest area available for the analyses of SW noble gases, especially important for the heavy noble gases.

Prior to Genesis mission, we had developed a unique method for collecting low-energy cometary volatiles by growing low-Z metal films on sapphire substrates (Hohenberg at al., 1997). This method utilized a technique we referred to as "active capture" and involved the "anomalous adsorption" of Xe and Kr at chemically active sites, permanently entrapping them in the growing metal films (Hohenberg et al., 2002). Anomalous adsorption is a term we use to distinguish the chemical bonding of heavy noble gases from conventional Van der Waals adsorption and requires the availability of unfilled bonds. In the course of refining the active capture technique, low background laser ablation methods were developed to extract noble gases from these films, during which backgrounds, trapping efficiencies and other properties of these films were extensively studied. A natural extension of this work led to the optimized Genesis SW collectors and recovery techniques of impinged SW gases.

The aluminum on Sapphire (AloS) collectors have many advantages over other thin films and over the polished aluminum collectors (PAC). First, Al has a relatively low melting point compared to other metallic films, requiring less laser power for ablation and therefore less energy deposited in the laser extraction cell which results in lower noble gas backgrounds (blanks), especially important for the low abundance heavy noble gases. Second, the low-Z of the target aluminum means that the backscatter of SW ions will be much smaller compared with other potential collectors such as Au, requiring a much smaller back scatter correction especially for the light noble gases where the projectile Z is also low. Third, aluminum is a good conductor, eliminating any charging effects. Finally, the rapid diffusion of hydrogen in Al (compared with Si and other collector materials) reduces lattice damage and lattice distortion effects caused by the huge amounts of SW hydrogen which can adversely affect the quantitative retention of the light noble gases. Moreover, these SW hydrogen effects are difficult to properly model or simulate so reducing the problem is the best way to minimize the effect (Meshik et al., 2000).

The main disadvantage of AloS is that the Al coating is somewhat fragile and can be easily damaged. Several scratched fragments of AloS have demonstrated measurable SW-He

Measuring the Isotopic Composition of Solar Wind Noble Gases 99

allowing a better depth resolution of SW noble gases and eliminating some of the massfractionation caused by diffusion in step-wise pyrolysis. An elegant version of CSSE was developed for Genesis gold SW-collectors which were step-wise amalgamated *in vacuo* by mercury vapor, thus incrementally releasing SW gases. This technique was also used in analysis of Genesis AuoS (gold film on sapphire) and the solid gold foils (Pepin et al., 2011). During the evolution of Genesis noble gas analyses, laser extraction techniques were further developed (Meshik et al., 2006) reducing the background even more for the PAC collectors. This provided an alternative to step-wise pyrolysis and CSSE. Gradually increasing the applied UV- laser power with each raster was thought to be capable of separating surface contamination from the more deeply implanted SW noble gases. However, there were considerable complications especially for the low abundant heavy noble gases which required the ablation of several square centimeters of the PAC SW collector to extract enough SW gases for precision measurements. During the process, sputtered aluminum from the collector was deposited both on the walls of the extraction cell and on the internal surface of the vacuum viewport, thus attenuating the laser power delivered to the sample while heating the viewport and the whole extraction cell. This progressively decreased the extraction efficiency of the laser and increased the noble gas background. During the course and evolution of these analyses several improvements were made which reduced the sputtering of collector material on the viewport (Figure 3) and the associated viewport

Fig. 3. Three laser extraction cells used for laser extraction of SW noble gases from Genesis Al-collectors. The X-stage moves perpendicular to the figure plane, the Y-stage moves from left to right. The angle between the laser beam and the normal to the ablated surface is /4 in (a), in (b, but the ablated Al does not reach the viewport) and /8 (c). The cells (a) and (c) were used for ablation of both AloS and PAC, cell (b) is only suitable for AloS and other

Genesis SW collectors with transparent sapphire substrates.

losses, especially for the light 3He isotope, making the remaining He isotopically heavier (Mabry, 2008).

The solid aluminum SW collector, PAC (Figure 2), is a harder material than the AloS film, thus it is somewhat more robust, but the excellent thermal conduction properties of solid aluminum requires a UV laser to adequately couple to the aluminum and a short-pulse to deliver the energy faster than the energy dissipation from the laser pit ("explosive" degassing with each laser pulse). This places some constraints on the laser system but it also provides means for better depth profiling of the released gases.

Fig. 2. Genesis Polished Aluminum Collector (PAC) cut in several fragments - (a). Fragment #4 was further split in two parts - (b), which were rearranged for UV-laser ablation and loaded into extraction cell - (c). Last panel (d) shows the PAC after analyses of noble gases released during the ablation.

## **3. Experimental**

## **3.1 Extraction of noble gases from Genesis solar wind collectors**

There are several ways of extracting noble gases from Genesis SW-collectors. The first is simple pyrolysis, melting of the material carrying the SW and extracting noble gases in a single step from the melt. The second is step-wise pyrolysis, increasing the temperature incrementally in steps, allowing extraction by enhanced diffusion, first from the weakly bound sites, usually the shallowly implanted noble gases (and most of the superficial contamination), then progressively from the more deeply implanted noble gases. This method was extensively used in the analyses of SW implanted into lunar soils and breccias, SW-rich (referred to as gas-rich) meteorites and the Apollo SWC-experiments. The third technique, mentioned above as CSSE (Wieler & Baur, 1994), which, in contrast with stepwise pyrolysis, is a step-wise etching method. This is carried out at constant temperature,

losses, especially for the light 3He isotope, making the remaining He isotopically heavier

The solid aluminum SW collector, PAC (Figure 2), is a harder material than the AloS film, thus it is somewhat more robust, but the excellent thermal conduction properties of solid aluminum requires a UV laser to adequately couple to the aluminum and a short-pulse to deliver the energy faster than the energy dissipation from the laser pit ("explosive" degassing with each laser pulse). This places some constraints on the laser system but it also

Fig. 2. Genesis Polished Aluminum Collector (PAC) cut in several fragments - (a). Fragment #4 was further split in two parts - (b), which were rearranged for UV-laser ablation and loaded into extraction cell - (c). Last panel (d) shows the PAC after analyses of noble gases

There are several ways of extracting noble gases from Genesis SW-collectors. The first is simple pyrolysis, melting of the material carrying the SW and extracting noble gases in a single step from the melt. The second is step-wise pyrolysis, increasing the temperature incrementally in steps, allowing extraction by enhanced diffusion, first from the weakly bound sites, usually the shallowly implanted noble gases (and most of the superficial contamination), then progressively from the more deeply implanted noble gases. This method was extensively used in the analyses of SW implanted into lunar soils and breccias, SW-rich (referred to as gas-rich) meteorites and the Apollo SWC-experiments. The third technique, mentioned above as CSSE (Wieler & Baur, 1994), which, in contrast with stepwise pyrolysis, is a step-wise etching method. This is carried out at constant temperature,

**3.1 Extraction of noble gases from Genesis solar wind collectors** 

provides means for better depth profiling of the released gases.

(Mabry, 2008).

released during the ablation.

**3. Experimental** 

allowing a better depth resolution of SW noble gases and eliminating some of the massfractionation caused by diffusion in step-wise pyrolysis. An elegant version of CSSE was developed for Genesis gold SW-collectors which were step-wise amalgamated *in vacuo* by mercury vapor, thus incrementally releasing SW gases. This technique was also used in analysis of Genesis AuoS (gold film on sapphire) and the solid gold foils (Pepin et al., 2011).

During the evolution of Genesis noble gas analyses, laser extraction techniques were further developed (Meshik et al., 2006) reducing the background even more for the PAC collectors. This provided an alternative to step-wise pyrolysis and CSSE. Gradually increasing the applied UV- laser power with each raster was thought to be capable of separating surface contamination from the more deeply implanted SW noble gases. However, there were considerable complications especially for the low abundant heavy noble gases which required the ablation of several square centimeters of the PAC SW collector to extract enough SW gases for precision measurements. During the process, sputtered aluminum from the collector was deposited both on the walls of the extraction cell and on the internal surface of the vacuum viewport, thus attenuating the laser power delivered to the sample while heating the viewport and the whole extraction cell. This progressively decreased the extraction efficiency of the laser and increased the noble gas background. During the course and evolution of these analyses several improvements were made which reduced the sputtering of collector material on the viewport (Figure 3) and the associated viewport

Fig. 3. Three laser extraction cells used for laser extraction of SW noble gases from Genesis Al-collectors. The X-stage moves perpendicular to the figure plane, the Y-stage moves from left to right. The angle between the laser beam and the normal to the ablated surface is /4 in (a), in (b, but the ablated Al does not reach the viewport) and /8 (c). The cells (a) and (c) were used for ablation of both AloS and PAC, cell (b) is only suitable for AloS and other Genesis SW collectors with transparent sapphire substrates.

Measuring the Isotopic Composition of Solar Wind Noble Gases 101

Precise isotopic analyses of light noble gases require a mass spectrometer with high resolving power (to separate 20Ne+ from 40Ar++ & HF+ and to separate 3He+ from HD+ & H3+) and a large dynamic range with minimal pressure effects. No commercially available mass spectrometer satisfied our requirements. Therefore, in this study we used a modified 90 magnetic sector mass-spectrometer, the "SuperGnome" (Hohenberg, 1980). It has highly sensitive GS-61 ion source (Baur, 1980), without electron focusing magnet, which results in an extremely small mass discrimination. Because the extraction fields are small and the ions originate on a cone leading to the same trajectory, all of ions have the same energy and follow nearly the same path. This leads to a tight cluster of trajectories, with no removal by source defining slits, so the instrument has a nearly 100% ion transmission. Since few ions are removed by the slits, it also has low memory effects and long useful counting times. In contrast, a widely used Nier-type ion source has significant and often non-reproducible isotope mass discrimination, lower sensitivity and typically 10-50% ion transmission, which

The disadvantage of GS-61 ion source is caused by the same things as its advantage. Since the ions originate in a region of low electric field gradient, they are slow at being extracted and then follow the same trajectory. Thus, when the pressure is higher the ion density is fairly large, causing space-charge effects. This effect is non-linear with sensitivity, transmission propensity for double charging caused by variable space charge effects in the ionization region. Space charge effects are not present for the heavy noble gases where ion density is low. However, for SW He and Ne, exacerbated by copious quantities of SW hydrogen, space charge effects can be severe and the extended time in the ionization region leads to significant and variable formation of 20NeH+ which interferes with the low abundance 21Ne+. The only way to correct for pressure effects in Ne and He measurements is to match the composition of sample to that of an independently known reference standard. In the course of this study we used artificial mixture of helium isotopes with 3He/4He = 6.5 10-4 (manufactured and certified by ChemGas, France), which is much closer to actual SW ratio than to atmospheric. This helium standard was mixed with atmospheric Ne-He to match SW He/Ne ratio, and with hydrogen to simulate the H abundance in these collectors. Mass resolution was set to ~ 200 so isobaric interferences from doubly ionized CO2++ and 40Ar++ were present on 22Ne+ and 20Ne+, respectively, the latter interference was significantly reduced by running the ion source at 48 eV electron energy. The doubly-charged correction factors were typically 40Ar++/Ar+ = 0.006 and CO2++/CO2+ = 0.02. The correction for interferences at 3He was more complex since it came from both HD+ and H3+ which are not present at constant proportion and, in fact, also pressure dependent. Luckily, after hydrogen removal, helium becomes the most abundant SW noble gas so the corrections for interference at m/e=3 never exceeded ~10% and the hydride corrections greatly reduced. 3He/4He and 21Ne/20Ne ratios were corrected for small effects due to high counting rates and deadtime, typically from 10 to 12 ns, corresponding to ~ 1% correction at a 1 MHz count

Argon analyses did not present any of these problems because Ar was cryogenically separated from He and Ne (and H) eliminating any of the pressure effects mentioned above. The SW 36Ar/38Ar ratio is close to the atmospheric value, and the terrestrial contamination can be accurately subtracted using 40Ar which is absent in SW in measurable amounts.

**3.3 Measurements of light noble gases** 

implies that 2 to 9 of every 10 ions are wasted.

rate.

heating, reducing the background. All of these improvements have one thing in common: since the sputtered material emerges as sin2 of the incidence angle it is highly weighted in a direction perpendicular to the collector surface. By allowing the laser to pass through the viewport and hit the surface at an oblique angle, the sputtered aluminum largely ends up on other parts of the extraction cell, harmlessly away from the vacuum viewport, as shown in the drawing (Figure 3).

## **3.2 Purification of noble gases prior to isotope analysis**

Although all of the Genesis collectors are made of ultrapure materials, some terrestrial heavy noble gases may be trapped at the interface between the sapphire substrate and the collector films. This can largely be avoided by refined extraction techniques which avoid that interface. Reduction of terrestrial contamination from material acquired during the hard landing was done by careful cleaning of the PAC to remove as much of this material as possible (Allton et al., 2006; Calaway et al., 2007). However, considering the fragility of the Al films, the AloS collectors were not extensively cleaned. The only surface treatment of the AloS was the mechanical removal of suspicious dust particles and water spots by repeated rinsing with acetone. Contamination was not a problem for the analyses of the abundant light noble gases, but for the less abundant Kr and Xe variable contamination resulted in backgrounds well above the blank levels observed in the same material not flown on the mission. One explanation for the elevated and non-reproducible behavior of the heavy noble gas background was a "brown stain", a thin Si-based polymerized coating, often observed on the flown collectors and other surfaces of the spacecraft (perhaps formed by UVpolymerization of surface contaminants). Ozone plasma treatments reduced Xe and Kr blank to some extent, but the "flown" and "not-flown" AloS collectors still had very different Kr and Xe backgrounds. It was first thought that some of the elevated blanks may have originated from the interface between the Al-film and sapphire substrate, mentioned previously but then why were the flight and non-flight materials so different?

It was finally realized that neither contamination by the "Utah mud" nor the "brown stain", could be responsible for the differences between the noble gas backgrounds of flight and the non-flight AloS collectors but it was the SW-hydrogen, the dominant SW component, that makes the major difference. The huge amounts of SW hydrogen, released during the Alablation of the AloS collector, interact with internal surfaces of the vacuum system and the getter material. Surface oxide removal and reduction of reacted getter alloy liberates significant quantities of noble gases which would otherwise be dormantly trapped there. Interestingly, ultrahigh vacuum systems are known to be efficiently cleaned with hydrogen at elevated temperatures. Therefore, any SW hydrogen implanted and now released from the AloS must be selectively removed from the vacuum system as quickly as possible to prevent excessive noble gas contamination by such surface "cleaning". To remove the hydrogen, we used a Pd finger, a 5 mm diameter Pd tube with 0.3 mm walls, with the interior exposed to the extraction system and the exterior exposed to the atmosphere, and this solved the problem. When the Pd tube was heated to 500C it removed 99% of hydrogen from the system to the atmosphere in less than 1 minute. It is interesting that oxygen at atmospheric pressure is needed on the exterior of the tube to remove the hydrogen. The Pd finger was the main modification of the noble gas purification line which otherwise is similar to that used in conventional noble gas mass spectrometry.

## **3.3 Measurements of light noble gases**

100 Exploring the Solar Wind

heating, reducing the background. All of these improvements have one thing in common: since the sputtered material emerges as sin2 of the incidence angle it is highly weighted in a direction perpendicular to the collector surface. By allowing the laser to pass through the viewport and hit the surface at an oblique angle, the sputtered aluminum largely ends up on other parts of the extraction cell, harmlessly away from the vacuum viewport, as shown in

Although all of the Genesis collectors are made of ultrapure materials, some terrestrial heavy noble gases may be trapped at the interface between the sapphire substrate and the collector films. This can largely be avoided by refined extraction techniques which avoid that interface. Reduction of terrestrial contamination from material acquired during the hard landing was done by careful cleaning of the PAC to remove as much of this material as possible (Allton et al., 2006; Calaway et al., 2007). However, considering the fragility of the Al films, the AloS collectors were not extensively cleaned. The only surface treatment of the AloS was the mechanical removal of suspicious dust particles and water spots by repeated rinsing with acetone. Contamination was not a problem for the analyses of the abundant light noble gases, but for the less abundant Kr and Xe variable contamination resulted in backgrounds well above the blank levels observed in the same material not flown on the mission. One explanation for the elevated and non-reproducible behavior of the heavy noble gas background was a "brown stain", a thin Si-based polymerized coating, often observed on the flown collectors and other surfaces of the spacecraft (perhaps formed by UVpolymerization of surface contaminants). Ozone plasma treatments reduced Xe and Kr blank to some extent, but the "flown" and "not-flown" AloS collectors still had very different Kr and Xe backgrounds. It was first thought that some of the elevated blanks may have originated from the interface between the Al-film and sapphire substrate, mentioned

previously but then why were the flight and non-flight materials so different?

similar to that used in conventional noble gas mass spectrometry.

It was finally realized that neither contamination by the "Utah mud" nor the "brown stain", could be responsible for the differences between the noble gas backgrounds of flight and the non-flight AloS collectors but it was the SW-hydrogen, the dominant SW component, that makes the major difference. The huge amounts of SW hydrogen, released during the Alablation of the AloS collector, interact with internal surfaces of the vacuum system and the getter material. Surface oxide removal and reduction of reacted getter alloy liberates significant quantities of noble gases which would otherwise be dormantly trapped there. Interestingly, ultrahigh vacuum systems are known to be efficiently cleaned with hydrogen at elevated temperatures. Therefore, any SW hydrogen implanted and now released from the AloS must be selectively removed from the vacuum system as quickly as possible to prevent excessive noble gas contamination by such surface "cleaning". To remove the hydrogen, we used a Pd finger, a 5 mm diameter Pd tube with 0.3 mm walls, with the interior exposed to the extraction system and the exterior exposed to the atmosphere, and this solved the problem. When the Pd tube was heated to 500C it removed 99% of hydrogen from the system to the atmosphere in less than 1 minute. It is interesting that oxygen at atmospheric pressure is needed on the exterior of the tube to remove the hydrogen. The Pd finger was the main modification of the noble gas purification line which otherwise is

the drawing (Figure 3).

**3.2 Purification of noble gases prior to isotope analysis** 

Precise isotopic analyses of light noble gases require a mass spectrometer with high resolving power (to separate 20Ne+ from 40Ar++ & HF+ and to separate 3He+ from HD+ & H3+) and a large dynamic range with minimal pressure effects. No commercially available mass spectrometer satisfied our requirements. Therefore, in this study we used a modified 90 magnetic sector mass-spectrometer, the "SuperGnome" (Hohenberg, 1980). It has highly sensitive GS-61 ion source (Baur, 1980), without electron focusing magnet, which results in an extremely small mass discrimination. Because the extraction fields are small and the ions originate on a cone leading to the same trajectory, all of ions have the same energy and follow nearly the same path. This leads to a tight cluster of trajectories, with no removal by source defining slits, so the instrument has a nearly 100% ion transmission. Since few ions are removed by the slits, it also has low memory effects and long useful counting times. In contrast, a widely used Nier-type ion source has significant and often non-reproducible isotope mass discrimination, lower sensitivity and typically 10-50% ion transmission, which implies that 2 to 9 of every 10 ions are wasted.

The disadvantage of GS-61 ion source is caused by the same things as its advantage. Since the ions originate in a region of low electric field gradient, they are slow at being extracted and then follow the same trajectory. Thus, when the pressure is higher the ion density is fairly large, causing space-charge effects. This effect is non-linear with sensitivity, transmission propensity for double charging caused by variable space charge effects in the ionization region. Space charge effects are not present for the heavy noble gases where ion density is low. However, for SW He and Ne, exacerbated by copious quantities of SW hydrogen, space charge effects can be severe and the extended time in the ionization region leads to significant and variable formation of 20NeH+ which interferes with the low abundance 21Ne+. The only way to correct for pressure effects in Ne and He measurements is to match the composition of sample to that of an independently known reference standard. In the course of this study we used artificial mixture of helium isotopes with 3He/4He = 6.5 10-4 (manufactured and certified by ChemGas, France), which is much closer to actual SW ratio than to atmospheric. This helium standard was mixed with atmospheric Ne-He to match SW He/Ne ratio, and with hydrogen to simulate the H abundance in these collectors.

Mass resolution was set to ~ 200 so isobaric interferences from doubly ionized CO2 ++ and 40Ar++ were present on 22Ne+ and 20Ne+, respectively, the latter interference was significantly reduced by running the ion source at 48 eV electron energy. The doubly-charged correction factors were typically 40Ar++/Ar+ = 0.006 and CO2++/CO2+ = 0.02. The correction for interferences at 3He was more complex since it came from both HD+ and H3+ which are not present at constant proportion and, in fact, also pressure dependent. Luckily, after hydrogen removal, helium becomes the most abundant SW noble gas so the corrections for interference at m/e=3 never exceeded ~10% and the hydride corrections greatly reduced. 3He/4He and 21Ne/20Ne ratios were corrected for small effects due to high counting rates and deadtime, typically from 10 to 12 ns, corresponding to ~ 1% correction at a 1 MHz count rate.

Argon analyses did not present any of these problems because Ar was cryogenically separated from He and Ne (and H) eliminating any of the pressure effects mentioned above. The SW 36Ar/38Ar ratio is close to the atmospheric value, and the terrestrial contamination can be accurately subtracted using 40Ar which is absent in SW in measurable amounts.

Measuring the Isotopic Composition of Solar Wind Noble Gases 103

B1, Z1 *136Xe 134Xe 132Xe 130Xe 128Xe 126Xe 124Xe* 

B3, Z3 *86Kr 84Kr 83Kr 82Kr 80Kr* 

Table 1. Assignment of Noblesse ion collectors for Measurements of Genesis heavy noble gases: All isotopes of Xe, Kr and Ar can be measured in five steps of the magnetic field (B) and associated zoom lens (Z) settings. At least one more step (not shown) is needed for the baseline measurement. All Kr isotopes (except 86Kr and 78Kr) are measured twice by different electron multipliers, providing an internal check for the multiplier performance.

passes through the outermost edge of the zoom lens and, when the electrostatic fringe field of the zoom lens is intentionally distorted, we can measure 78Kr slightly off-center, where the contribution of benzene is more negligible (Figure 4). This distortion does not affect the other Kr isotopes which pass through the middle part of the lens, but it does provide means for partially resolving the benzene interference and obtaining a valid 78Kr measurement.

Fig. 4. Simultaneous detection of Kr isotopes 84, 83, 82, 80 and 78. Intentional fringe field distortion of the electrostatic zoom lens allows measurement of 78Kr without significant benzene contribution. Vertical scales (count rates) are different for different isotopes. Horizontal axis is mass for central (axial) isotope, 82Kr in this case. The assignment of ion

However, even after solution of the benzene interference problem at 78Kr, measuring all heavy noble gas isotopes in one run without separation remained difficult because of the limited dynamic range of the miniature electron multipliers and because of the pressure effects in the ion source (although these were much less for the Nier-type ion source in Noblesse than for the GS-61 ion source in the SuperGnome). Additionally, there is the

B4, Z4 *84Kr 83Kr 82Kr 80Kr 78Kr*  B5, Z5 *40Ar 38Ar 37Cl 36Ar* 

B2, Z2 *131Xe 129Xe* 

Switching from one step to another takes less than 2 seconds.

collectors corresponds to step 4 of Table 1.

Ion collectors (EM – electron multipliers, FC – Faraday Cup): FC EM1 EM2 EM3 EM4 EM5 EM6 EM7 EM8

Magnet, Zoom Lens

Isobaric interferences from HCℓ on m/e= 36 and 38 were very small and, since all of our ultrahigh vacuum pumping lines used oil free scroll pumps, magnetically levitated (lubricant-free) turbomolecular pumps and ion pumps, eliminating most hydrocarbon interferences, the ubiquitous hydrocarbon interference at m/e=38 was not present. All of these factors contributed to the fact that our first measured SW 36Ar/38Ar ratio was 5.501 ± 0.005 (1σ) (Meshik et al., 2007) which remains the most accurate value for SW argon measured to date.

## **3.4 Measurements of heavy noble gases**

To measure the isotopic compositions of the heavy noble gases, which are present in such low abundances in the solar wind, a special multi-collector version of the Noblesse mass spectrometer was specially constructed for us by NU Instruments. It utilized a "bright", Nier-type, ion source with ~70% ion transmission and unique fast electrostatic zoom lens allowing us to change the effective spacing between isotopes. Since different noble gases have different spacing between the isotopes on the focal plane, variable isotope spacing allows us to use a multiple-dynode collector system to simultaneously measure isotopes of different noble gases. Eight continuous dynode electron multipliers from Burle™, and one Faraday cup collector on the high mass side provided for the simultaneous counting of 9 different ion beams. The high sensitivity of this instrument, 1.8 10-16 cm3 STP 132Xe/cps is ~ 3-times higher than that of the SuperGnome, and the 8 multipliers, made this instrument ideal for the low count rate measurements of Genesis SW Kr and Xe. Moreover, the zoom lens allowed Kr and Xe to be measured simultaneously. However, the miniature Burle electron multipliers are mounted just few mm apart, allowing no room for electrostatic shielding so they do suffer from some crosstalk with > 50,000 count/s ion beams. This configuration is, therefore, not as suitable for He and Ne when a high dynamic range is more important but, for the heavy noble gases, when the counting statistic represents the major source of errors, Multi-Noblesse excels.

The Noblesse mass spectrometer has a counting half-life for Xe of ~ 17 minutes, almost 3 times shorter than SuperGnome instrument, reflecting its higher sensitivity, and its Niertype source makes memory effects more pronounced in the Noblesse. To minimize these effects, and to correct for them, only small spikes of atmospheric Xe and Kr were ever admitted into this mass spectrometer for calibration and all vacuum lines, extraction, purification and pumping systems were assembled from new parts which were never exposed to any isotopically anomalous noble gases. Whenever possible, these parts were internally electropolished to minimize isobaric contaminations and pumping lines were made as short as possible with no pipes being thinner than ¾" in diameter for maximum conductance. Additionally, the high voltage power supply for the ion source was modified to be switched on simultaneously with the beginning of measurement, providing a more precise "time zero" when the gas inside the mass spectrometer has not been yet altered by counting and memory growth. The configuration of ion collector for heavy noble gas measurements is shown in Table 1.

There is a potential problem associated with hydrocarbon interference at m/e = 78 due to the omnipresent C6H6 (benzene) which is not completely resolved from 78Kr. Attempts to correct for benzene using hydrocarbons measured at m/e=79 and 77, which were measured anyway, (step 4 in Table 1), were not successful. Luckily 78Kr, the lightest stable Kr isotope,

Isobaric interferences from HCℓ on m/e= 36 and 38 were very small and, since all of our ultrahigh vacuum pumping lines used oil free scroll pumps, magnetically levitated (lubricant-free) turbomolecular pumps and ion pumps, eliminating most hydrocarbon interferences, the ubiquitous hydrocarbon interference at m/e=38 was not present. All of these factors contributed to the fact that our first measured SW 36Ar/38Ar ratio was 5.501 ± 0.005 (1σ) (Meshik et al., 2007) which remains the most accurate value for SW argon

To measure the isotopic compositions of the heavy noble gases, which are present in such low abundances in the solar wind, a special multi-collector version of the Noblesse mass spectrometer was specially constructed for us by NU Instruments. It utilized a "bright", Nier-type, ion source with ~70% ion transmission and unique fast electrostatic zoom lens allowing us to change the effective spacing between isotopes. Since different noble gases have different spacing between the isotopes on the focal plane, variable isotope spacing allows us to use a multiple-dynode collector system to simultaneously measure isotopes of different noble gases. Eight continuous dynode electron multipliers from Burle™, and one Faraday cup collector on the high mass side provided for the simultaneous counting of 9 different ion beams. The high sensitivity of this instrument, 1.8 10-16 cm3 STP 132Xe/cps is ~ 3-times higher than that of the SuperGnome, and the 8 multipliers, made this instrument ideal for the low count rate measurements of Genesis SW Kr and Xe. Moreover, the zoom lens allowed Kr and Xe to be measured simultaneously. However, the miniature Burle electron multipliers are mounted just few mm apart, allowing no room for electrostatic shielding so they do suffer from some crosstalk with > 50,000 count/s ion beams. This configuration is, therefore, not as suitable for He and Ne when a high dynamic range is more important but, for the heavy noble gases, when the counting statistic represents the

The Noblesse mass spectrometer has a counting half-life for Xe of ~ 17 minutes, almost 3 times shorter than SuperGnome instrument, reflecting its higher sensitivity, and its Niertype source makes memory effects more pronounced in the Noblesse. To minimize these effects, and to correct for them, only small spikes of atmospheric Xe and Kr were ever admitted into this mass spectrometer for calibration and all vacuum lines, extraction, purification and pumping systems were assembled from new parts which were never exposed to any isotopically anomalous noble gases. Whenever possible, these parts were internally electropolished to minimize isobaric contaminations and pumping lines were made as short as possible with no pipes being thinner than ¾" in diameter for maximum conductance. Additionally, the high voltage power supply for the ion source was modified to be switched on simultaneously with the beginning of measurement, providing a more precise "time zero" when the gas inside the mass spectrometer has not been yet altered by counting and memory growth. The configuration of ion collector for heavy noble gas

There is a potential problem associated with hydrocarbon interference at m/e = 78 due to the omnipresent C6H6 (benzene) which is not completely resolved from 78Kr. Attempts to correct for benzene using hydrocarbons measured at m/e=79 and 77, which were measured anyway, (step 4 in Table 1), were not successful. Luckily 78Kr, the lightest stable Kr isotope,

measured to date.

**3.4 Measurements of heavy noble gases** 

major source of errors, Multi-Noblesse excels.

measurements is shown in Table 1.


Table 1. Assignment of Noblesse ion collectors for Measurements of Genesis heavy noble gases: All isotopes of Xe, Kr and Ar can be measured in five steps of the magnetic field (B) and associated zoom lens (Z) settings. At least one more step (not shown) is needed for the baseline measurement. All Kr isotopes (except 86Kr and 78Kr) are measured twice by different electron multipliers, providing an internal check for the multiplier performance. Switching from one step to another takes less than 2 seconds.

passes through the outermost edge of the zoom lens and, when the electrostatic fringe field of the zoom lens is intentionally distorted, we can measure 78Kr slightly off-center, where the contribution of benzene is more negligible (Figure 4). This distortion does not affect the other Kr isotopes which pass through the middle part of the lens, but it does provide means for partially resolving the benzene interference and obtaining a valid 78Kr measurement.

Fig. 4. Simultaneous detection of Kr isotopes 84, 83, 82, 80 and 78. Intentional fringe field distortion of the electrostatic zoom lens allows measurement of 78Kr without significant benzene contribution. Vertical scales (count rates) are different for different isotopes. Horizontal axis is mass for central (axial) isotope, 82Kr in this case. The assignment of ion collectors corresponds to step 4 of Table 1.

However, even after solution of the benzene interference problem at 78Kr, measuring all heavy noble gas isotopes in one run without separation remained difficult because of the limited dynamic range of the miniature electron multipliers and because of the pressure effects in the ion source (although these were much less for the Nier-type ion source in Noblesse than for the GS-61 ion source in the SuperGnome). Additionally, there is the

Measuring the Isotopic Composition of Solar Wind Noble Gases 105

Fig. 5. Isotopic composition of light noble gases extracted from PAC using stepped power UV-laser ablation. Argon in each step has been corrected for the atmospheric contamination assuming that 40Ar is absent in the SW. He and Ne are not corrected, since there is no way to determine the atmospheric contribution to each individual step. Numbers indicate laser output in mJ, R is re-raster with the same power, P stands for pyrolysis made after the completion of laser ablation. Dashed lines show sum of all steps in Ar plot and bulk IR-

Similar patterns were reported for 20Ne/22Ne ratios measured by the CSSE extractions from the BMG (Bulk Metallic Glass, Zr58.5Nb2.8Cu15.6Ni12.8Al10.3) Genesis SW collector (Grimberg et al., 2006). Analogous isotopic effect has been observed in SW He, Ne and Ar in the lunar regolith samples (e.g. Benkert et al., 1993) and, as we discussed in the Introduction, was interpreted at that time as indicating the presence of two distinct solar wind components: (1) Conventional solar wind, SW, and (2) A more energetic, thus more deeply implanted, high energy tail of the solar wind, referred to as Solar Energetic Particles, SEP. Until recently this interpretation was widely accepted, and even became incorporated into noble gas text books (Ozima & Podosek., 2002; Noble Gases in Geochemistry and Cosmochemistry, 2002) as a distinct component. Very few papers (e.g. Becker, 1995) recognized that solar wind isotope ratios will naturally get heavier with implantation depth. Genesis results clearly support this

Argon was extracted from PAC using 23 steps of UV-laser ablation with some on them being repeat extractions made at the same output laser power. These were the first analyses made using Noblesse multi-collector mass spectrometer. A record low value for the 40Ar/36Ar ratio of 1.12 was found in step #16. This is the most pure SW-Ar (lowest 40Ar/36Ar ratio) ever observed for a natural sample, demonstrating the ability of the laser stepped-

ablation values of AloS. Error bars are 1σ.

realization and a distinct SEP component is no longer necessary.

"change-of-charge" effect that interferes with the measurement of 80Kr. As mentioned in 3.3, doubly charged 40Ar++ interferes with singly charged 20Ne+ but another effect of doubly charged 40Ar++ interferes with 80Kr. A small fraction of 40Ar++ ions can pick up an electron from the source defining slits, becoming 40Ar+ but with the double energy, thus following the same trajectory as 80Kr+. This effect is clearly detectible whenever Kr is measured in the presence of 40Ar. Therefore, Ar must be cryogenically separated from Kr, although complete Ar removal cannot be achieved without losing a small fraction of Kr and fractionating the rest. At a temperature of -125C for activated charcoal trap ~2% of the original Ar is still present so an additional measure is required to further reduce the "change-of-charge" effect on 80Kr. This was done by a reduction of the electron energy from 100 eV to 75 eV at the cost of ~ 10% sensitivity loss. Luckily, the solar wind contains very little, if any, 40Ar so most of the "change-of-charge" problems occur during the calibration of the mass spectrometer.

## **4. Results and discussion**

## **4.1 Depth profiles of light noble gases**

Measuring the composition of noble gases as a function of implantation depth required a uniform laser ablation of the same area of SW collector with each step incrementally increased in the power density delivered to the target. Our frequency quadrupled NdYAG laser (Powerlite-6030 from Continuum™) delivered ~ 10 mJ of 266 nm in 7ns pulses at 30Hz. The best power stability (shot-to-shot) of 12% (barely sufficient for depth profiling) was achieved only at maximum power and only after about a ½-hour "warm-up" period. Several methods were used to control the power: From a pair of rotatable polarizers to attenuating the output power by series of parallel fused quartz plates, each reflecting a few percent of incident beam. However, best results were achieved by selecting delay times of from 125 ms to 300 ms between the flash lamp and the Pockels cell varying the oscillator cavity gain curve of the NdYAG rod.

During the UV-laser step profiling, the laser remained stationary while the extraction cell, mounted on a X-Y-stage moved back and forth (Fig. 3). The stage was programmed to keep velocity constant (typically, 3 mm/s). A fast shutter (computer controlled) blocks the laser during the U-turn of the stage to prevent the power density delivered at the edges of rastered area from increasing beyond that delivered elsewhere. All of the computer codes to control the shutter and the Newport stage via GPIB interface were written in Labview 7.1.

To avoid any contribution of noble gases from the walls of the ablation pit due to stage instability or from beam bleed, with the potential for heating of the un-degassed aluminum adjacent to the rastered area as power increases, each subsequent raster area was made progressively smaller. Therefore, the gas amounts were normalized to the area specific for each step. An example of a completed stepped-power UV laser extraction is shown at the top of the Figure 2d. Depth profiles for He, Ne (preliminarily reported by Meshik et al., 2006; Mabry et al., 2007) and Ar (this work) are assembled in Figure 5.

Solar wind ions are bound to the solar magnetic field and, thus, all ions are implanted with equal velocity so that all SW noble gases (Figure 5) show the same general pattern: The lighter isotopes of each gas (3He, 20Ne, 21Ne and 36Ar) are implanted at shallower depths than the heavier isotopes (4He, 22Ne, and 38Ar), in general agreement with SRIM-2008 simulations for ions implanted at the same velocity, therefore at slightly different energies.

"change-of-charge" effect that interferes with the measurement of 80Kr. As mentioned in 3.3, doubly charged 40Ar++ interferes with singly charged 20Ne+ but another effect of doubly charged 40Ar++ interferes with 80Kr. A small fraction of 40Ar++ ions can pick up an electron from the source defining slits, becoming 40Ar+ but with the double energy, thus following the same trajectory as 80Kr+. This effect is clearly detectible whenever Kr is measured in the presence of 40Ar. Therefore, Ar must be cryogenically separated from Kr, although complete Ar removal cannot be achieved without losing a small fraction of Kr and fractionating the rest. At a temperature of -125C for activated charcoal trap ~2% of the original Ar is still present so an additional measure is required to further reduce the "change-of-charge" effect on 80Kr. This was done by a reduction of the electron energy from 100 eV to 75 eV at the cost of ~ 10% sensitivity loss. Luckily, the solar wind contains very little, if any, 40Ar so most of the "change-of-charge" problems occur during the calibration of the mass spectrometer.

Measuring the composition of noble gases as a function of implantation depth required a uniform laser ablation of the same area of SW collector with each step incrementally increased in the power density delivered to the target. Our frequency quadrupled NdYAG laser (Powerlite-6030 from Continuum™) delivered ~ 10 mJ of 266 nm in 7ns pulses at 30Hz. The best power stability (shot-to-shot) of 12% (barely sufficient for depth profiling) was achieved only at maximum power and only after about a ½-hour "warm-up" period. Several methods were used to control the power: From a pair of rotatable polarizers to attenuating the output power by series of parallel fused quartz plates, each reflecting a few percent of incident beam. However, best results were achieved by selecting delay times of from 125 ms to 300 ms between the flash lamp and the Pockels cell varying the oscillator cavity gain

During the UV-laser step profiling, the laser remained stationary while the extraction cell, mounted on a X-Y-stage moved back and forth (Fig. 3). The stage was programmed to keep velocity constant (typically, 3 mm/s). A fast shutter (computer controlled) blocks the laser during the U-turn of the stage to prevent the power density delivered at the edges of rastered area from increasing beyond that delivered elsewhere. All of the computer codes to control the shutter and the Newport stage via GPIB interface were written in Labview 7.1. To avoid any contribution of noble gases from the walls of the ablation pit due to stage instability or from beam bleed, with the potential for heating of the un-degassed aluminum adjacent to the rastered area as power increases, each subsequent raster area was made progressively smaller. Therefore, the gas amounts were normalized to the area specific for each step. An example of a completed stepped-power UV laser extraction is shown at the top of the Figure 2d. Depth profiles for He, Ne (preliminarily reported by Meshik et al.,

Solar wind ions are bound to the solar magnetic field and, thus, all ions are implanted with equal velocity so that all SW noble gases (Figure 5) show the same general pattern: The lighter isotopes of each gas (3He, 20Ne, 21Ne and 36Ar) are implanted at shallower depths than the heavier isotopes (4He, 22Ne, and 38Ar), in general agreement with SRIM-2008 simulations for ions implanted at the same velocity, therefore at slightly different energies.

2006; Mabry et al., 2007) and Ar (this work) are assembled in Figure 5.

**4. Results and discussion** 

curve of the NdYAG rod.

**4.1 Depth profiles of light noble gases** 

Fig. 5. Isotopic composition of light noble gases extracted from PAC using stepped power UV-laser ablation. Argon in each step has been corrected for the atmospheric contamination assuming that 40Ar is absent in the SW. He and Ne are not corrected, since there is no way to determine the atmospheric contribution to each individual step. Numbers indicate laser output in mJ, R is re-raster with the same power, P stands for pyrolysis made after the completion of laser ablation. Dashed lines show sum of all steps in Ar plot and bulk IRablation values of AloS. Error bars are 1σ.

Similar patterns were reported for 20Ne/22Ne ratios measured by the CSSE extractions from the BMG (Bulk Metallic Glass, Zr58.5Nb2.8Cu15.6Ni12.8Al10.3) Genesis SW collector (Grimberg et al., 2006). Analogous isotopic effect has been observed in SW He, Ne and Ar in the lunar regolith samples (e.g. Benkert et al., 1993) and, as we discussed in the Introduction, was interpreted at that time as indicating the presence of two distinct solar wind components: (1) Conventional solar wind, SW, and (2) A more energetic, thus more deeply implanted, high energy tail of the solar wind, referred to as Solar Energetic Particles, SEP. Until recently this interpretation was widely accepted, and even became incorporated into noble gas text books (Ozima & Podosek., 2002; Noble Gases in Geochemistry and Cosmochemistry, 2002) as a distinct component. Very few papers (e.g. Becker, 1995) recognized that solar wind isotope ratios will naturally get heavier with implantation depth. Genesis results clearly support this realization and a distinct SEP component is no longer necessary.

Argon was extracted from PAC using 23 steps of UV-laser ablation with some on them being repeat extractions made at the same output laser power. These were the first analyses made using Noblesse multi-collector mass spectrometer. A record low value for the 40Ar/36Ar ratio of 1.12 was found in step #16. This is the most pure SW-Ar (lowest 40Ar/36Ar ratio) ever observed for a natural sample, demonstrating the ability of the laser stepped-

Measuring the Isotopic Composition of Solar Wind Noble Gases 107

from the pyrolysis of the PAC which had already been degased by stepped-power laser extractions. Total melting was initially carried out to ensure that all of the SW noble gases had been completely removed by the laser extraction but, as it turned out, this was not the case. Several percent of the SW gases remained present in the PAC even after laser extraction to a depth much greater than the solar wind implantation. UV-laser has sufficient power to extract almost 100% of noble gases from PAC in one extraction step, but when the power increased step-by-step the extraction is no longer complete. About 3.4% of 36Ar, 6.8% of 20Ne and 8.3% of 4He are still present in PAC after 23-steps of laser extraction (step P in Figure 5) and released only by total pyrolysis of the remaining piece. Interestingly, this is more than it could be expected from SRIM simulations (Figure 1) of the solar wind implantation: so this is an extraction effect, not only an implantation effect. Microscopic observations of laser rastered areas of PAC show that the laser raster did not really make an excavation with a flat bottom, but melted, and re-melted the Al several times, evaporating only a part of it. This heating causes enhanced diffusion of gases in the melt and, since light gases move faster than the heavy ones, more He goes into the remaining Al than Ne (and Ar). In other words, heating from the stepped-power laser technique modifies the original distribution of SW noble gases, making the profile wider and deeper with each step so, in this sense, the technique has some properties similar to traditional step-wise pyrolysis. Therefore, the interpretation of stepped-power laser extractions is not as straightforward as we would like it to be because of the modification of the distribution by the extraction itself and perhaps some fractionation effects since the implanted light noble gases are more easily mobilized. One way to reduce the problem is to use laser pulses much shorter than the 7 ns

used in this work to more explosively degas the material without as much heating.

profiles can be used to estimate SW-Ne losses in real Genesis materials.

The degree of diffusion losses of noble gases in Genesis SW collectors depends on the material and the thermal history. A step-wise pyrolysis is indicative of such losses. Figure 7 shows the cumulative release of 20Ne implanted at 20 keV into the different Genesis materials: PAC, AloS and BMG. The first two materials, Al alloy and pure Al, are significantly less retentive compared to amorphous (below~1000°C) BMG. These Ne release

20Ne/22Ne ratios and fluxes of SW 20Ne, measured in the St. Louis, Minnesota and Zürich laboratories, are shown on Figure 8. Although all measured Ne isotope ratios agree to within 3σ, there is a trend suggesting that the higher 20Ne/22Ne ratios seem to correspond to the higher 20Ne fluxes, and the PAC seems to suggest a lower 20Ne flux than either the AloS, BMG or CZ-Si. Given the different thermal diffusion properties of the Genesis collectors (Figure. 7), this seems to make sense. Since exposure times were identical, the lower apparent SW-Ne fluxes indicate some loss of SW Ne. If such losses do occur, the lighter isotope, 20Ne in this case, will escape preferentially for two reasons: (1) it is implanted at shallower depth and, (2) since it is lighter, it is slightly more mobile than 22Ne, thus more susceptible to diffusive loss. Moreover, broadening of the original depth distribution will be more significant for 20Ne than for 22Ne. This has been confirmed by comparison of two fragments of PAC Genesis sample analyzed at different conditions. One was unbaked prior to analysis, another was kept in vacuum for 10 days at 220C resulting in a lower 20Ne content and a lower 20Ne/22Ne ratio. A long-term He diffusion experiment in which a sample of PAC was baked at 240C for 322 days (38% of the duration of the Genesis collector exposure time) showed large losses of He, confirming significant diffusive losses of light

power technique to separate SW-Ar from terrestrial contamination, mainly present at the surface of the SW-collector. The total SW 36Ar/38Ar = 5.496 ± 0.011 (calculated as weighted sum of all steps) is indistinguishable from 36Ar/38Ar = 5.501 ± 0.005 measured in AloS using one step IR-laser extraction (Meshik et al., 2007). Considering that these two measurements were made two years apart using two different mass spectrometers and two different laser extraction techniques, this agreement gives us strong confidence that this is a true SW-Ar composition. Both of these SW 36Ar/38Ar analyses agree well with SW-Ar measured independently in different Genesis collectors: AuoS (Gold on Sapphire), DOS (Diamondlike-carbon on Sapphire) and CZ-Si (Czochralski-grown Si). The timeline of SW Ar measurements (Figure 6) demonstrates the high precision of the Genesis results compared to all of the pre-Genesis measurements. Only after Genesis we can confidently conclude that the SW 36Ar/38Ar ratio is significantly higher than that in the terrestrial atmosphere, suggesting atmospheric losses in the early evolution of the Earth's atmosphere.

Fig. 6. Comparison of pre-Genesis analyses of SW 36Ar/38Ar ratios: (a) (Cerutti, 1974; Benkert et al., 1993; Weygand et al., 2001; Palma et al., 2002; Geiss et al., 2004) with Genesis measurements (Meshik et al., 2007; Grimberg et al., 2008; Heber et al., 2009; Vogel et al., 2001; Pepin et al., 2012; this work). Abbreviations next to Genesis data points stand for laboratories were the analyses were performed: WU – Washington University, ETH – Eidgenössischen Technischen Hochschule Zürich, UM – University of Minnesota. All Genesis results, except for the early, ETH analysis, agree with each other and demonstrate significantly higher precision than those based upon pre-Genesis data. SW 36Ar fluxes at L1 station (b) are measured in different Genesis targets by different laboratories. All error bars are 1σ.

Solar wind 36Ar fluxes are in reasonable, although not perfect, agreement (all are within 3σ). Interestingly, the lower values of SW 36Ar fluxes are found in metal films (Al and Au) while the 8% higher fluxes are observed in nonmetallic materials (Figure 6). Future experiments will show if this difference is real or an experimental artifact.

The stepped-power laser extraction techniques were developed and refined during the evolution of these Genesis analyses and some of the properties of these techniques were realized only after the experiment was completed. One interesting observation was made

power technique to separate SW-Ar from terrestrial contamination, mainly present at the surface of the SW-collector. The total SW 36Ar/38Ar = 5.496 ± 0.011 (calculated as weighted sum of all steps) is indistinguishable from 36Ar/38Ar = 5.501 ± 0.005 measured in AloS using one step IR-laser extraction (Meshik et al., 2007). Considering that these two measurements were made two years apart using two different mass spectrometers and two different laser extraction techniques, this agreement gives us strong confidence that this is a true SW-Ar composition. Both of these SW 36Ar/38Ar analyses agree well with SW-Ar measured independently in different Genesis collectors: AuoS (Gold on Sapphire), DOS (Diamondlike-carbon on Sapphire) and CZ-Si (Czochralski-grown Si). The timeline of SW Ar measurements (Figure 6) demonstrates the high precision of the Genesis results compared to all of the pre-Genesis measurements. Only after Genesis we can confidently conclude that the SW 36Ar/38Ar ratio is significantly higher than that in the terrestrial atmosphere,

suggesting atmospheric losses in the early evolution of the Earth's atmosphere.

Fig. 6. Comparison of pre-Genesis analyses of SW 36Ar/38Ar ratios: (a) (Cerutti, 1974; Benkert et al., 1993; Weygand et al., 2001; Palma et al., 2002; Geiss et al., 2004) with Genesis measurements (Meshik et al., 2007; Grimberg et al., 2008; Heber et al., 2009; Vogel et al., 2001; Pepin et al., 2012; this work). Abbreviations next to Genesis data points stand for laboratories were the analyses were performed: WU – Washington University, ETH – Eidgenössischen Technischen Hochschule Zürich, UM – University of Minnesota. All Genesis results, except for the early, ETH analysis, agree with each other and demonstrate significantly higher precision than those based upon pre-Genesis data. SW 36Ar fluxes at L1 station (b) are measured in different Genesis targets by different laboratories. All error bars

Solar wind 36Ar fluxes are in reasonable, although not perfect, agreement (all are within 3σ). Interestingly, the lower values of SW 36Ar fluxes are found in metal films (Al and Au) while the 8% higher fluxes are observed in nonmetallic materials (Figure 6). Future experiments

The stepped-power laser extraction techniques were developed and refined during the evolution of these Genesis analyses and some of the properties of these techniques were realized only after the experiment was completed. One interesting observation was made

will show if this difference is real or an experimental artifact.

are 1σ.

from the pyrolysis of the PAC which had already been degased by stepped-power laser extractions. Total melting was initially carried out to ensure that all of the SW noble gases had been completely removed by the laser extraction but, as it turned out, this was not the case. Several percent of the SW gases remained present in the PAC even after laser extraction to a depth much greater than the solar wind implantation. UV-laser has sufficient power to extract almost 100% of noble gases from PAC in one extraction step, but when the power increased step-by-step the extraction is no longer complete. About 3.4% of 36Ar, 6.8% of 20Ne and 8.3% of 4He are still present in PAC after 23-steps of laser extraction (step P in Figure 5) and released only by total pyrolysis of the remaining piece. Interestingly, this is more than it could be expected from SRIM simulations (Figure 1) of the solar wind implantation: so this is an extraction effect, not only an implantation effect. Microscopic observations of laser rastered areas of PAC show that the laser raster did not really make an excavation with a flat bottom, but melted, and re-melted the Al several times, evaporating only a part of it. This heating causes enhanced diffusion of gases in the melt and, since light gases move faster than the heavy ones, more He goes into the remaining Al than Ne (and Ar). In other words, heating from the stepped-power laser technique modifies the original distribution of SW noble gases, making the profile wider and deeper with each step so, in this sense, the technique has some properties similar to traditional step-wise pyrolysis. Therefore, the interpretation of stepped-power laser extractions is not as straightforward as we would like it to be because of the modification of the distribution by the extraction itself and perhaps some fractionation effects since the implanted light noble gases are more easily mobilized. One way to reduce the problem is to use laser pulses much shorter than the 7 ns used in this work to more explosively degas the material without as much heating.

The degree of diffusion losses of noble gases in Genesis SW collectors depends on the material and the thermal history. A step-wise pyrolysis is indicative of such losses. Figure 7 shows the cumulative release of 20Ne implanted at 20 keV into the different Genesis materials: PAC, AloS and BMG. The first two materials, Al alloy and pure Al, are significantly less retentive compared to amorphous (below~1000°C) BMG. These Ne release profiles can be used to estimate SW-Ne losses in real Genesis materials.

20Ne/22Ne ratios and fluxes of SW 20Ne, measured in the St. Louis, Minnesota and Zürich laboratories, are shown on Figure 8. Although all measured Ne isotope ratios agree to within 3σ, there is a trend suggesting that the higher 20Ne/22Ne ratios seem to correspond to the higher 20Ne fluxes, and the PAC seems to suggest a lower 20Ne flux than either the AloS, BMG or CZ-Si. Given the different thermal diffusion properties of the Genesis collectors (Figure. 7), this seems to make sense. Since exposure times were identical, the lower apparent SW-Ne fluxes indicate some loss of SW Ne. If such losses do occur, the lighter isotope, 20Ne in this case, will escape preferentially for two reasons: (1) it is implanted at shallower depth and, (2) since it is lighter, it is slightly more mobile than 22Ne, thus more susceptible to diffusive loss. Moreover, broadening of the original depth distribution will be more significant for 20Ne than for 22Ne. This has been confirmed by comparison of two fragments of PAC Genesis sample analyzed at different conditions. One was unbaked prior to analysis, another was kept in vacuum for 10 days at 220C resulting in a lower 20Ne content and a lower 20Ne/22Ne ratio. A long-term He diffusion experiment in which a sample of PAC was baked at 240C for 322 days (38% of the duration of the Genesis collector exposure time) showed large losses of He, confirming significant diffusive losses of light

Measuring the Isotopic Composition of Solar Wind Noble Gases 109

Fig. 8. Solar wind 20Ne/22Ne ratios (a) and 20Ne fluxes at L1 station (b) measured by different laboratories in different Genesis SW-collector materials. All abbreviations are the same as in Fig. 6. "Lunar" SW-Ne is from Benkert et al., 1993. A diffusion experiment demonstrates that PAC kept in vacuum for 10 days at 220C may lose some Ne,

Helium is the most abundant noble gas in the SW. It is also the lightest, the most susceptible to diffusive loss and, because it has the largest relative difference in masses of its two isotopes, it is the most indicative of isotopic mass fractionation. All Genesis He analyses and some "pre"- Genesis results are shown in Figure 9. Both isotope ratios and apparent SW He fluxes are scattered much more than would be justified by the statistical uncertainties.

Fig. 9. Isotopic compositions (a) and fluxes (b) at the L1 point measured in different Genesis SW collectors in St. Louis, Minnesota and Zürich labs. Lunar and Apollo SWC SW data are from Benkert et al., 1993 and Geiss et al., 2004. The 500 day average He composition from Ulysses/SWICS is from Bodmer et al., 1995. Aluminum collectors (both AloS and PAC) are from Mabry et al., 2007 and Mabry., 2009. PAC (baked) was kept at 240°C for 322 days, ~38% of total Genesis collection time. Helium losses from all Al-collectors are evident and

apparently are accompanied by preferential losses of 3He.

preferentially 20Ne.

Fig. 7. Release profiles of Ne implanted into different SW collectors at 1 keV/nucleon. Each temperature step was maintained for 30 min. The difference in release curves is the basis for estimation of thermal gas losses and the average temperatures experienced by the Genesis collectors.

noble gases from the PAC although, in that experiment, Ne was not measured (Mabry, 2009). These observations, and the verifying experiments, all point out that some Ne losses, and consequent isotope fractionation, must have occurred with the PAC collector. Although the "low" 20Ne/22Ne ratios observed in the PAC agree more closely with the previous "lunar" ratios (cf. Benkert et al., 1993), we believe the higher 20Ne/22Ne ratios observed in the AloS collectors, being less modified; provide a better measurement of the modern solar wind. Given a solar wind flux of about 107 protons/cm2/s, lunar surface material is quickly saturated with solar wind hydrogen to the point that, without extensive diffusive redistribution, the implanted solar wind hydrogen atoms will outnumber the host lattice atoms in a broad region near the end of its range in only a few tens of years. This means, among other things, extensive lattice damage and enhanced surface erosion with associated effects on the diffusion and retention of the implanted light noble gases. We, therefore, expect large and variable diffusive losses from lunar soils and regolith samples. In addition, even though the foils were carefully cleaned, the Apollo Solar Wind Composition Experiment is still susceptible to contamination by fine lunar dust that contains both diffusively modified solar wind Ne and spallation-produced Ne. Thus, we conclude that AloS, CZ-Si and DOS measurements should provide the definitive composition of the modern solar wind Ne. However, the Ne measured in the Zurich laboratory in the Si and DOS collectors, which are expected to be equally retentive, appear to be slightly heavier than those measured in the AloS collectors (St. Louis) and in the AuoS collectors (Minneapolis), as shown in Figure 8. At present time we do not have a reasonable explanation. However, a higher resolution (~1500) mass-spectrometer is expected to be installed at Washington University in the future, it will be capable of resolving 40Ar++ from 20Ne+, removing one of the uncertainties in Ne analysis. Re-analysis of Genesis SW neon using this instrument will provide an opportunity for better precision and exploration of any apparent discrepancy in the SW 20Ne/22Ne ratios obtained by the different laboratories.

Fig. 7. Release profiles of Ne implanted into different SW collectors at 1 keV/nucleon. Each temperature step was maintained for 30 min. The difference in release curves is the basis for estimation of thermal gas losses and the average temperatures experienced by the Genesis

noble gases from the PAC although, in that experiment, Ne was not measured (Mabry, 2009). These observations, and the verifying experiments, all point out that some Ne losses, and consequent isotope fractionation, must have occurred with the PAC collector. Although the "low" 20Ne/22Ne ratios observed in the PAC agree more closely with the previous "lunar" ratios (cf. Benkert et al., 1993), we believe the higher 20Ne/22Ne ratios observed in the AloS collectors, being less modified; provide a better measurement of the modern solar wind. Given a solar wind flux of about 107 protons/cm2/s, lunar surface material is quickly saturated with solar wind hydrogen to the point that, without extensive diffusive redistribution, the implanted solar wind hydrogen atoms will outnumber the host lattice atoms in a broad region near the end of its range in only a few tens of years. This means, among other things, extensive lattice damage and enhanced surface erosion with associated effects on the diffusion and retention of the implanted light noble gases. We, therefore, expect large and variable diffusive losses from lunar soils and regolith samples. In addition, even though the foils were carefully cleaned, the Apollo Solar Wind Composition Experiment is still susceptible to contamination by fine lunar dust that contains both diffusively modified solar wind Ne and spallation-produced Ne. Thus, we conclude that AloS, CZ-Si and DOS measurements should provide the definitive composition of the modern solar wind Ne. However, the Ne measured in the Zurich laboratory in the Si and DOS collectors, which are expected to be equally retentive, appear to be slightly heavier than those measured in the AloS collectors (St. Louis) and in the AuoS collectors (Minneapolis), as shown in Figure 8. At present time we do not have a reasonable explanation. However, a higher resolution (~1500) mass-spectrometer is expected to be installed at Washington University in the future, it will be capable of resolving 40Ar++ from 20Ne+, removing one of the uncertainties in Ne analysis. Re-analysis of Genesis SW neon using this instrument will provide an opportunity for better precision and exploration of any apparent discrepancy in the SW 20Ne/22Ne ratios obtained by the different laboratories.

collectors.

Fig. 8. Solar wind 20Ne/22Ne ratios (a) and 20Ne fluxes at L1 station (b) measured by different laboratories in different Genesis SW-collector materials. All abbreviations are the same as in Fig. 6. "Lunar" SW-Ne is from Benkert et al., 1993. A diffusion experiment demonstrates that PAC kept in vacuum for 10 days at 220C may lose some Ne, preferentially 20Ne.

Helium is the most abundant noble gas in the SW. It is also the lightest, the most susceptible to diffusive loss and, because it has the largest relative difference in masses of its two isotopes, it is the most indicative of isotopic mass fractionation. All Genesis He analyses and some "pre"- Genesis results are shown in Figure 9. Both isotope ratios and apparent SW He fluxes are scattered much more than would be justified by the statistical uncertainties.

Fig. 9. Isotopic compositions (a) and fluxes (b) at the L1 point measured in different Genesis SW collectors in St. Louis, Minnesota and Zürich labs. Lunar and Apollo SWC SW data are from Benkert et al., 1993 and Geiss et al., 2004. The 500 day average He composition from Ulysses/SWICS is from Bodmer et al., 1995. Aluminum collectors (both AloS and PAC) are from Mabry et al., 2007 and Mabry., 2009. PAC (baked) was kept at 240°C for 322 days, ~38% of total Genesis collection time. Helium losses from all Al-collectors are evident and apparently are accompanied by preferential losses of 3He.

Measuring the Isotopic Composition of Solar Wind Noble Gases 111

collectors reacted with the getter material. Since the SAES getters were produced by sintering in an inert atmosphere, this liberated dormant Xe and Kr from the getters. The quantity of hydrogen was so large that it could not be separated cryogenically in the sample system. Removing hydrogen from the flight tube using palladium (described in 3.2) significantly reduced this problem, but did not eliminate it completely, so new techniques

The alternative approach for blank correction is based on the significant difference in 84Kr/132Xe ratios between the terrestrial atmosphere (27.78; Ozima & Podosek., 2002) and the solar wind (9.55, Meshik et al., 2009). In the case of binary mixtures of SW and terrestrial components, the 84Kr/132Xe can be used as a measure of terrestrial contribution. Since the Washington University multi-collector mass spectrometer, the laser extraction cells and the purification system have never seen any isotopically anomalous gases, we are limited to these two compositions (with negligible mass fractionation). A capability to simultaneously measure both heavy noble gases in a single run (3.4) was needed to use this approach, but this was the plan all along. Laser extraction was done in a single step using maximal power to ensure the complete extraction of SW noble gases, to provide maximum signal and to minimize the analysis time. Xe and Kr were cryogenically separated from at least 98% of the Ar using activated "Berkeley" charcoal finger kept at -125C, which reduced the change-ofcharge effect at 80Kr. Both PAC and AloS collectors were analyzed in different laser cells (shown in Figure 3) using pulsed laser extraction at two wavelengths: 266 nm for PAC and 1064 nm for AloS. It was realized that Kr and Xe may not be trapped in atmospheric proportion, with Xe usually more "sticky" than Kr, but it was assumed that they would probably not be isotopically fractionated to any significant degree (an assumption that could be checked later). To determine the actual trapped 84Kr/132Xe ratio we assumed that, for all 24 samples analyzed, this ratio was constant. Equations (1) and (2) describe binary mixtures

for minimizing terrestrial noble gases had to be developed.

between SW and terrestrial trapped gases for each measurement:

�� �� � �

�� ��� � �

� � �� ��� �� �� � �

� � �� �� �� ��� � �

Here SW refers to Solar Wind, M to Measured and T to Trapped (or Terrestrial) and the two unknowns are (132Xe/84Kr)SW and (132Xe/84Kr)T. With two equations only two measurements are needed to determine the values for these ratios but, for the 24 measurements available, the system is over-determined. A multi-variance solution is obtained from minimization of the standard deviations of the SW fluencies and the most probable values for (132Xe/84Kr)SW, and correspondingly, 132XeSW and 84KrSW were obtained. The best convergence, shown in Figure 10a, was achieved at (84Kr/132Xe)trapped = 24.4 (Figure 10b), only 12% lower than the terrestrial atmosphere, a value confirming our assumption of no significant isotopic mass

All Kr and Xe isotopic analyses are shown in Figures 11 and 12, respectively, and Table 2 presents final results. All of the data in these figures show consistent results even though

� �� �� ��� �� �� � �

� �� �� �� �� ��� � �

� � �� ��� �� �� �

� � �� �� �� ��� �

� �� (2)

� � (1)

��

��

����

���� ��� � ���

fractionation in this component.

�� � ��� <sup>×</sup> �� �� �� ���

��� × �� �� ��

Systematical errors, not reflected in the data, evidently exist in this figure. Both types of Al collectors have significantly lower concentrations of SW-He (Figure 9b), demonstrating the diffusive losses expected from the thermal release profiles shown in Figure 7. The AloS and the PAC have the lowest 3He/4He ratios observed among the SW collectors and pre-Genesis SW-He determinations, suggesting that aluminum diffusively loses He at the temperature of the exposed collector surfaces and, as expected from the diffusive properties shown in Figure 7, the PAC loses more He than the AloS collector.

The real-time diffusion experiment conducted by Mabry (2009) confirms the poor SW-He retention properties of the PAC at the elevated collection temperature. Both the 3He/4He ratios and the He concentrations (reflecting apparent He fluxes) were significantly lower following a 322 day vacuum bake (38% of the Genesis mission) at 240°C. Even the unbaked reference sample of the PAC demonstrates the lowest measured apparent He fluxes and 3He/4He ratios indicating significant He losses during the Genesis collection period, not surprising since the temperatures of Genesis PAC and AloS collectors were estimated to be around 165°C (Mabry, 2009). Therefore, none of the Genesis aluminum collectors completely retain solar wind He or preserve the original 3He/4He ratios, both can only be considered as lower limits. Among the other SW-He collectors, DOS (Diamond-like Carbon on Sapphire) CZ-Si (Czochralski-grown silicon) and gold (both AuoS and foil), DOS is probably the best choice since it does not require as high backscattering corrections (up to 35% for Au). AloS however do not demonstrate significant Ne losses and completely retain SW Ar. Therefore, AloS was the choice material for analyses of SW heavy noble gases.

## **4.2 Heavy noble gases**

The large concentration of the light SW noble gases He and Ne in the Genesis collectors meant that corrections for atmospheric or other contaminations were usually negligible. Argon from the collectors contained significant terrestrial contributions but since the solar wind has negligible 40Ar, and the terrestrial isotopic ratios are well known, this can easily be removed to leave pure solar wind argon. For krypton and xenon, which are far less abundant in the solar wind, the terrestrial contamination becomes a serious problem and there is no "terrestrial only" isotope to identify the trapped component. In fact, the compositions of SW and terrestrial noble gases are not significantly different so partitioning by isotopic composition is not possible. Our original intention was to use stepped-power laser extraction to separate any superficial surface-correlated contamination from the more deeply implanted SW noble gases. A complicating factor is the low abundances of the heavy noble gases in the SW which requires analyzing very large areas of the collectors for precise measurements in stepped-power laser extractions. The conventional way to document terrestrial Xe and Kr contributions is to analyze reference (non-flight) SW collectors, manufactured in the same way as the flight collectors, utilizing the same procedures and raster areas. The Xe and Kr signals measured in these non-flight coupons would then be a proxy for blanks in the actual collectors. Unfortunately, the AloS collectors were manufactured in several batches and after the "hard" landing of the Genesis return capsule it became challenging to pair flight and non-flight AloS material. That said, a more severe problem was found. In the laser extraction experiments it was observed that the Xe and Kr blanks were neither proportional to the raster areas nor were they very reproducible. It was soon realized that the large quantities of implanted SW hydrogen released from SW

Systematical errors, not reflected in the data, evidently exist in this figure. Both types of Al collectors have significantly lower concentrations of SW-He (Figure 9b), demonstrating the diffusive losses expected from the thermal release profiles shown in Figure 7. The AloS and the PAC have the lowest 3He/4He ratios observed among the SW collectors and pre-Genesis SW-He determinations, suggesting that aluminum diffusively loses He at the temperature of the exposed collector surfaces and, as expected from the diffusive properties shown in

The real-time diffusion experiment conducted by Mabry (2009) confirms the poor SW-He retention properties of the PAC at the elevated collection temperature. Both the 3He/4He ratios and the He concentrations (reflecting apparent He fluxes) were significantly lower following a 322 day vacuum bake (38% of the Genesis mission) at 240°C. Even the unbaked reference sample of the PAC demonstrates the lowest measured apparent He fluxes and 3He/4He ratios indicating significant He losses during the Genesis collection period, not surprising since the temperatures of Genesis PAC and AloS collectors were estimated to be around 165°C (Mabry, 2009). Therefore, none of the Genesis aluminum collectors completely retain solar wind He or preserve the original 3He/4He ratios, both can only be considered as lower limits. Among the other SW-He collectors, DOS (Diamond-like Carbon on Sapphire) CZ-Si (Czochralski-grown silicon) and gold (both AuoS and foil), DOS is probably the best choice since it does not require as high backscattering corrections (up to 35% for Au). AloS however do not demonstrate significant Ne losses and completely retain SW Ar. Therefore,

The large concentration of the light SW noble gases He and Ne in the Genesis collectors meant that corrections for atmospheric or other contaminations were usually negligible. Argon from the collectors contained significant terrestrial contributions but since the solar wind has negligible 40Ar, and the terrestrial isotopic ratios are well known, this can easily be removed to leave pure solar wind argon. For krypton and xenon, which are far less abundant in the solar wind, the terrestrial contamination becomes a serious problem and there is no "terrestrial only" isotope to identify the trapped component. In fact, the compositions of SW and terrestrial noble gases are not significantly different so partitioning by isotopic composition is not possible. Our original intention was to use stepped-power laser extraction to separate any superficial surface-correlated contamination from the more deeply implanted SW noble gases. A complicating factor is the low abundances of the heavy noble gases in the SW which requires analyzing very large areas of the collectors for precise measurements in stepped-power laser extractions. The conventional way to document terrestrial Xe and Kr contributions is to analyze reference (non-flight) SW collectors, manufactured in the same way as the flight collectors, utilizing the same procedures and raster areas. The Xe and Kr signals measured in these non-flight coupons would then be a proxy for blanks in the actual collectors. Unfortunately, the AloS collectors were manufactured in several batches and after the "hard" landing of the Genesis return capsule it became challenging to pair flight and non-flight AloS material. That said, a more severe problem was found. In the laser extraction experiments it was observed that the Xe and Kr blanks were neither proportional to the raster areas nor were they very reproducible. It was soon realized that the large quantities of implanted SW hydrogen released from SW

Figure 7, the PAC loses more He than the AloS collector.

AloS was the choice material for analyses of SW heavy noble gases.

**4.2 Heavy noble gases** 

collectors reacted with the getter material. Since the SAES getters were produced by sintering in an inert atmosphere, this liberated dormant Xe and Kr from the getters. The quantity of hydrogen was so large that it could not be separated cryogenically in the sample system. Removing hydrogen from the flight tube using palladium (described in 3.2) significantly reduced this problem, but did not eliminate it completely, so new techniques for minimizing terrestrial noble gases had to be developed.

The alternative approach for blank correction is based on the significant difference in 84Kr/132Xe ratios between the terrestrial atmosphere (27.78; Ozima & Podosek., 2002) and the solar wind (9.55, Meshik et al., 2009). In the case of binary mixtures of SW and terrestrial components, the 84Kr/132Xe can be used as a measure of terrestrial contribution. Since the Washington University multi-collector mass spectrometer, the laser extraction cells and the purification system have never seen any isotopically anomalous gases, we are limited to these two compositions (with negligible mass fractionation). A capability to simultaneously measure both heavy noble gases in a single run (3.4) was needed to use this approach, but this was the plan all along. Laser extraction was done in a single step using maximal power to ensure the complete extraction of SW noble gases, to provide maximum signal and to minimize the analysis time. Xe and Kr were cryogenically separated from at least 98% of the Ar using activated "Berkeley" charcoal finger kept at -125C, which reduced the change-ofcharge effect at 80Kr. Both PAC and AloS collectors were analyzed in different laser cells (shown in Figure 3) using pulsed laser extraction at two wavelengths: 266 nm for PAC and 1064 nm for AloS. It was realized that Kr and Xe may not be trapped in atmospheric proportion, with Xe usually more "sticky" than Kr, but it was assumed that they would probably not be isotopically fractionated to any significant degree (an assumption that could be checked later). To determine the actual trapped 84Kr/132Xe ratio we assumed that, for all 24 samples analyzed, this ratio was constant. Equations (1) and (2) describe binary mixtures between SW and terrestrial trapped gases for each measurement:

$${}^{\otimes 4}Kr\_{SW} = {}^{\otimes 4}Kr\_M \times \left[ \left( \frac{^{132}Xe}{^{\otimes 4}Kr} \right)\_M - \left( \frac{^{132}Xe}{^{\otimes 4}Kr} \right)\_\tau \right] \Bigg/ \left[ \left( \frac{^{132}Xe}{^{\otimes 4}Kr} \right)\_\tau - \left( \frac{^{132}Xe}{^{\otimes 4}Kr} \right)\_{SW} \right] \tag{1}$$

$${}^{132}Xe\_{SW} = {}^{132}Xe\_M \times \left[ \left( \frac{{}^{84}Kr}{{}^{132}Xe} \right)\_M - \left( \frac{{}^{84}Kr}{{}^{132}Xe} \right)\_T \right] \Bigg/ \left[ \left( \frac{{}^{84}Kr}{{}^{132}Kr} \right)\_T - \left( \frac{{}^{84}Kr}{{}^{132}Xe} \right)\_{SW} \right] \tag{2}$$

Here SW refers to Solar Wind, M to Measured and T to Trapped (or Terrestrial) and the two unknowns are (132Xe/84Kr)SW and (132Xe/84Kr)T. With two equations only two measurements are needed to determine the values for these ratios but, for the 24 measurements available, the system is over-determined. A multi-variance solution is obtained from minimization of the standard deviations of the SW fluencies and the most probable values for (132Xe/84Kr)SW, and correspondingly, 132XeSW and 84KrSW were obtained. The best convergence, shown in Figure 10a, was achieved at (84Kr/132Xe)trapped = 24.4 (Figure 10b), only 12% lower than the terrestrial atmosphere, a value confirming our assumption of no significant isotopic mass fractionation in this component.

All Kr and Xe isotopic analyses are shown in Figures 11 and 12, respectively, and Table 2 presents final results. All of the data in these figures show consistent results even though

Measuring the Isotopic Composition of Solar Wind Noble Gases 113

Fig. 11. Kr isotopic composition measured in Genesis Al collectors. Fitting line forced trough the estimated trapped component, the ordinate intercept gives isotopic composition of the solar wind. Different colors correspond to different experimental conditions, which within

statistical errors result in the same SW composition.

Fig. 10. The best convergence of SW fluencies (a) has been achieved at 132Xe/84Kr = 0.041 (b), providing our current fluence estimate: (1.15 ± 0.04) 106 132Xe atoms/cm2 and (1.08 ± 0.05) 107 84Kr atoms/cm2.


Table 2. Isotopic composition of heavy noble gases in solar wind measured in aluminum Genesis collectors. Errors are 1σ.

they represent both types of aluminum SW collectors, were analyzed in different extraction cells under different conditions, using two different pulsed laser wavelengths, and were performed several months apart.

The isotopic composition of solar wind heavy noble gases from the Genesis collectors (this work) can be compared with solar wind Xe and Kr previously inferred from lunar surface material (c.f. Pepin et al., 1995, Figure 13).

A few first-order observations can be made: The isotopic ratios of heavy SW noble gases implanted by lunar regolith over millions of years are indistinguishable from the contemporary SW observed in the Genesis collectors to within < 1%. This sets an upper limit to possible temporal variations of SW Kr and Xe. The small isotope differences we do observe suggest that SW-Kr inferred from the lunar regolith is slightly heavier than that we measure in Genesis while no such trend is observed for Xe. SW compositions inferred from lunar regolith may be more subjective to systematic error, and they are less precise than those measured by Genesis, at least at the major isotopes. For instance, the trend we see in this comparison is suggestive of some diffusive loss of Kr from the lunar regolith, not the case for the more retentive Xe. However, in order to test whether this effect is real, Kr compositions inferred from the lunar regolith should be revisited.

Fig. 10. The best convergence of SW fluencies (a) has been achieved at 132Xe/84Kr = 0.041 (b), providing our current fluence estimate: (1.15 ± 0.04) 106 132Xe atoms/cm2 and (1.08 ± 0.05)

86Kr 84Kr 83Kr 82Kr 80Kr 78Kr .3012 (4) ≡ 1 .2034 (2) .2054 (2) .0412 (2) .00642 (5)

136Xe 134Xe 132Xe 131Xe 130Xe 129Xe 128Xe 126Xe 124Xe .3003 (6) .3692 (7) ≡ 1 .8263 (13) .1649 (4) 1.0401(10) .0842 (3) .00417(9) .00492 (7)

Table 2. Isotopic composition of heavy noble gases in solar wind measured in aluminum

they represent both types of aluminum SW collectors, were analyzed in different extraction cells under different conditions, using two different pulsed laser wavelengths, and were

The isotopic composition of solar wind heavy noble gases from the Genesis collectors (this work) can be compared with solar wind Xe and Kr previously inferred from lunar surface

A few first-order observations can be made: The isotopic ratios of heavy SW noble gases implanted by lunar regolith over millions of years are indistinguishable from the contemporary SW observed in the Genesis collectors to within < 1%. This sets an upper limit to possible temporal variations of SW Kr and Xe. The small isotope differences we do observe suggest that SW-Kr inferred from the lunar regolith is slightly heavier than that we measure in Genesis while no such trend is observed for Xe. SW compositions inferred from lunar regolith may be more subjective to systematic error, and they are less precise than those measured by Genesis, at least at the major isotopes. For instance, the trend we see in this comparison is suggestive of some diffusive loss of Kr from the lunar regolith, not the case for the more retentive Xe. However, in order to test whether this effect is real, Kr

107 84Kr atoms/cm2.

Genesis collectors. Errors are 1σ.

performed several months apart.

material (c.f. Pepin et al., 1995, Figure 13).

compositions inferred from the lunar regolith should be revisited.

Fig. 11. Kr isotopic composition measured in Genesis Al collectors. Fitting line forced trough the estimated trapped component, the ordinate intercept gives isotopic composition of the solar wind. Different colors correspond to different experimental conditions, which within statistical errors result in the same SW composition.

Measuring the Isotopic Composition of Solar Wind Noble Gases 115

Fig. 13. "Lunar" SW (Pepin at al, 1995) vs. Genesis SW (this work). Although normalized to the Genesis composition as a delta plot, the errors shown (1σ) are not propagated, so

Depth profile of solar wind argon retrieved from Genesis Al collector confirms the isotopic fractionation which occurs during implantation at constant velocity, an effect previously observed for helium and neon. This eliminates the need for a unique heavy SEP noble gas component thought to be required from analyses of lunar surface material. New, more precise Ar analyses, and recent results from two independent laboratories, confirm our earlier published value for the 36Ar/38Ar ratio in the solar wind. The unique analytical capability developed at Washington University to simultaneously analyze all Xe and Kr isotopes allows us to determine the composition of heavy noble gases in the solar wind to a

We are in debt to Judith Allton, Patti Burkett, Phil Freedman, Amy Jurewicz, Karen McNamara, Melissa Rodriguez, John Saxton, Eileen Stansbery, Rainer Wieler, Roger Wiens Dorothy Woolum, and all members of Genesis Science Team for their invaluable help and

Allton, JH., Calaway, MJ., Hittle, JD., Rodriguez, MC., Stansbery, EK., & McNamara, KM.

megasonic flow on Genesis array collectors. Lunar Planet. Sci. XXXVII, 2324

(2006) Cleaning surface particle contamination with ultrapure water (UPW)

precision exceeding previous values inferred from lunar surface material.

support. This work was supported by NASA grant NNX07AM76G.

overlapping errors infer consistency.

**5. Conclusion** 

**6. Acknowledgments** 

**7. References** 

Fig. 12. Xe isotopic composition in Genesis Aluminum collectors. Fitting line forced trough the estimated trapped component, ordinate intercept gives isotopic composition of the solar wind. Different colors correspond to different experimental conditions.

Fig. 13. "Lunar" SW (Pepin at al, 1995) vs. Genesis SW (this work). Although normalized to the Genesis composition as a delta plot, the errors shown (1σ) are not propagated, so overlapping errors infer consistency.

## **5. Conclusion**

114 Exploring the Solar Wind

Fig. 12. Xe isotopic composition in Genesis Aluminum collectors. Fitting line forced trough the estimated trapped component, ordinate intercept gives isotopic composition of the solar

wind. Different colors correspond to different experimental conditions.

Depth profile of solar wind argon retrieved from Genesis Al collector confirms the isotopic fractionation which occurs during implantation at constant velocity, an effect previously observed for helium and neon. This eliminates the need for a unique heavy SEP noble gas component thought to be required from analyses of lunar surface material. New, more precise Ar analyses, and recent results from two independent laboratories, confirm our earlier published value for the 36Ar/38Ar ratio in the solar wind. The unique analytical capability developed at Washington University to simultaneously analyze all Xe and Kr isotopes allows us to determine the composition of heavy noble gases in the solar wind to a precision exceeding previous values inferred from lunar surface material.

## **6. Acknowledgments**

We are in debt to Judith Allton, Patti Burkett, Phil Freedman, Amy Jurewicz, Karen McNamara, Melissa Rodriguez, John Saxton, Eileen Stansbery, Rainer Wieler, Roger Wiens Dorothy Woolum, and all members of Genesis Science Team for their invaluable help and support. This work was supported by NASA grant NNX07AM76G.

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Olinger, C., Reisenfeld, DB., Alton, J., Basten, R., McNamara, K., Stansbery, E., & Bernett, DS. (2007) Refinement and implications of noble gas measurements from Genesis. *Proceedings of 38th Lunar and Planetary Science Conference, Houston, Texas*,

659-669

10351-3)

Abstract #2412

Abstract #5083

318, 433-435

*Texas*, Abstract #2037

Report. Available from


Most extraterrestrial materials discovered on the Earth have no solar wind noble gases. In fact, only four types of extraterrestrial materials contain noble gases attributed to the solar wind or its fractionated component: gas-rich meteorites, lunar materials collected by the Apollo missions, asteroid samples returned from Itokawa by the Hayabusa mission, and micrometeorites. Except for micrometeorites, all of these have a specific history of solar wind irradiation on the surface of their parent bodies. On the other hand, solar wind noble gases in micrometeorites are implanted during orbital evolution in interplanetary space. Micrometeorites have a different origin and irradiation history from the other three materials and from typical meteorites, meaning that these tiny particles that fell on the Earth can provide us valuable information about the activity of the solar system. Of all the analytical methods in planetary science, noble gas analysis of extraterrestrial materials is one of the most useful, because the analysis can reveal not only their origin and age but also their history of irradiation by galactic and solar cosmic rays and solar wind. In particular, the most reliable positive proof of an extraterrestrial origin for micrometeorites is the solar wind noble gases. In this chapter, solar wind noble gases trapped in micrometeorites are

First, the nomenclature of extraterrestrial dust must be explained because the peculiar technical terms in the field of planetary science are perplexing for researchers belonging to different scientific fields. The main terms for extraterrestrial dust are *micrometeorite*, *interplanetary dust particle* (*IDP*), *cosmic spherule*, and *cosmic dust*. *Micrometeorite* can indicate all types of extraterrestrial dust collected on the Earth, but is mainly used to indicate extraterrestrial dust corrected in polar regions. *IDPs* are very small dust particles (<30 μm in diameter) collected in the stratosphere by airplane and are often called *stratospheric dust particles* or *Brownlee particles*. *Cosmic spherules* are small spherical particles recovered from deep-sea sediment, polar regions, and sedimentary rocks. Their spherical shape is due to severe heating during atmospheric entry. Tiny spherical particles found in sedimentary rocks are generally called *microspherules*, *microkrystite*, or *microtektites*. *Unmelted micrometeorites* indicates micrometeorites other than the cosmic spherules, whose shape is irregular. *Cosmic dust* indicates all types of extraterrestrial dust, including intergalactic dust, interstellar dust, interplanetary dust, and circumplanetary dust. *Extraterrestrial dust* is

**1. Introduction** 

reviewed.

**1.1 Nomenclature of extraterrestrial dust** 

Takahito Osawa

*Japan* 

*Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA)* 

Ziegler, JF., Biersack, JP., & Ziegler, MD. (2008) SRIM - The Stopping and Range of Ions in Matter. 398p. ISBN-13 978-0-9654207-1-6 **6** 

## Takahito Osawa

*Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA) Japan* 

## **1. Introduction**

120 Exploring the Solar Wind

Ziegler, JF., Biersack, JP., & Ziegler, MD. (2008) SRIM - The Stopping and Range of Ions in

Most extraterrestrial materials discovered on the Earth have no solar wind noble gases. In fact, only four types of extraterrestrial materials contain noble gases attributed to the solar wind or its fractionated component: gas-rich meteorites, lunar materials collected by the Apollo missions, asteroid samples returned from Itokawa by the Hayabusa mission, and micrometeorites. Except for micrometeorites, all of these have a specific history of solar wind irradiation on the surface of their parent bodies. On the other hand, solar wind noble gases in micrometeorites are implanted during orbital evolution in interplanetary space. Micrometeorites have a different origin and irradiation history from the other three materials and from typical meteorites, meaning that these tiny particles that fell on the Earth can provide us valuable information about the activity of the solar system. Of all the analytical methods in planetary science, noble gas analysis of extraterrestrial materials is one of the most useful, because the analysis can reveal not only their origin and age but also their history of irradiation by galactic and solar cosmic rays and solar wind. In particular, the most reliable positive proof of an extraterrestrial origin for micrometeorites is the solar wind noble gases. In this chapter, solar wind noble gases trapped in micrometeorites are reviewed.

## **1.1 Nomenclature of extraterrestrial dust**

First, the nomenclature of extraterrestrial dust must be explained because the peculiar technical terms in the field of planetary science are perplexing for researchers belonging to different scientific fields. The main terms for extraterrestrial dust are *micrometeorite*, *interplanetary dust particle* (*IDP*), *cosmic spherule*, and *cosmic dust*. *Micrometeorite* can indicate all types of extraterrestrial dust collected on the Earth, but is mainly used to indicate extraterrestrial dust corrected in polar regions. *IDPs* are very small dust particles (<30 μm in diameter) collected in the stratosphere by airplane and are often called *stratospheric dust particles* or *Brownlee particles*. *Cosmic spherules* are small spherical particles recovered from deep-sea sediment, polar regions, and sedimentary rocks. Their spherical shape is due to severe heating during atmospheric entry. Tiny spherical particles found in sedimentary rocks are generally called *microspherules*, *microkrystite*, or *microtektites*. *Unmelted micrometeorites* indicates micrometeorites other than the cosmic spherules, whose shape is irregular. *Cosmic dust* indicates all types of extraterrestrial dust, including intergalactic dust, interstellar dust, interplanetary dust, and circumplanetary dust. *Extraterrestrial dust* is

(Theil and Schmidt, 1961; Shima and Yabuki, 1968; Maurette et al., 1986, 1987, 1991; Koeberl and Hagen, 1989; Cresswell and Herd, 1992; Taylor et al., 1997, 1998; Nakamura et al., 1999; Yada and Kojima, 2000; Iwata and Imae, 2002; Rochette et al., 2008; Carole et al., 2011). Antarctic micrometeorites (AMMs) have larger sizes (50–300 μm) than the IDPs captured in the stratosphere (<30 μm). Since most of the mass accreted by the Earth is contained in larger particles (50–400 μm) (Kortenkamp and Dermott, 1998), AMMs represent the

Isotopic noble gas study on micrometeorites was difficult for a long time because of terrestrial contamination and the small sizes of micrometeorites. Measurements on single cosmic particles had to wait for great improvement of analytical devices. Therefore, the first noble gas study on micrometeorites was a measurement on deep-sea sediments in which micrometeorites were concentrated. The first noble gas isotopic study on deep-sea sediments was performed by Merrihue (1964). Magnetic and nonmagnetic separates of modern red clays from the Pacific Ocean were analyzed using a glass extraction and purification system, and excess 3He and 21Ne were discovered. The reported 3He/4He ratios (shown as 4He/3He in Merrihue's paper) are clearly higher than that of the terrestrial atmosphere, and a relatively high 20Ne/22Ne ratio (11.0 ± 1.0) is reported in the 1000°C step of the magnetic separate. 40Ar/36Ar ratios lower than that of the atmosphere in the 1000°C and 1400°C steps of the magnetic separate (268 ± 7 and 172 ± 8) were clearly detected, indicating the presence of extraterrestrial materials. This excellent research for the first time presented overwhelming evidence that extraterrestrial materials with extraterrestrial noble gases had accumulated in the deep-sea sediments. Nine years later, Krylov et al. (1973) reported He isotopic compositions of fifteen oceanic oozes recovered from various regions of the Pacific and Atlantic oceans and the iceberg-melting region of Greenland, which were analyzed by researchers in the Soviet Union. The isotopic ratios for Pacific red clays are tens or a hundred times that found in the various crustal rocks. On the other hand, Atlantic red clays have low 3He/4He ratios of 2–3 × 10−6 and no 3He anomaly was found in the Greenland samples. They believed that the likely source for the elevated 3He content in the Pacific Ocean sediments is cosmic rather than the hypothetical 3He from the mantle in the clays. The idea was confirmed by studying nitric-acid–treated ooze, which had the same order of 3He/4He ratios as untreated ooze. Indeed, the high 3He/4He ratios found in the red

After these two reports, research in the field stagnated for a long time, and these important researches were forgotten completely. Japanese researchers, however, renewed study in the field in the 1980s. Ozima et al. (1984) measured thirty-nine sediments from twelve different sites, ten sites from the western to central Pacific and two sites from the Atlantic Ocean. They found 3He/4He ratios higher than 5 × 10−5 for six sites and concluded that the very high 3He/4He ratios in the sediments reflected the input of extraterrestrial materials. Amari and Ozima (1985) subsequently reported a He anomaly in deep-sea sediments, and they rediscovered that the carrier of exotic He was concentrated in magnetic fractions, which was consistent with the result of Merrihue's analysis. Since most terrestrial particles are nonmagnetic, magnetic cosmic dusts are concentrated in magnetic separates. They concluded that the ferromagnetic separates are essentially magnetite using thermomagnetic

interplanetary dust population well.

**2. Solar wind noble gases in deep-sea sediment** 

clays should be attributed to micrometeorites.

another versatile term synonymous with *cosmic dust*, but it is not as widely used as *cosmic dust*.

*Micrometeorite* is thought to be the best term representing extraterrestrial dust in this chapter for a few reasons. First, the cosmic dusts with solar wind noble gases reviewed here are not intergalactic dusts or interstellar dusts. Second, the Antarctic micrometeorites that are the main target of this paper are not IDPs. Therefore, the word *micrometeorite* adequately represents all types of cosmic dust that contain solar wind noble gases.

## **1.2 Collection of micrometeorites**

It was already suspected in the Middle Ages that a large number of dusty objects existed in interplanetary space. Zodiacal light is a faint glow that extends away from the sun in the ecliptic plane of the sky, visible to the naked eye in the western sky shortly after sunset or in the eastern sky shortly before sunrise. Already in 1683, Giovanni Domenico Cassini presented the correct explanation of this prominent light phenomenon visible to the human eye. Its spectrum indicates it to be sunlight scattered by interplanetary dust orbiting the sun. It is called "counter-glow" or "Gegenschein" in German (Yamakoshi, 1994). The zodiacal light contributes about a third of the total light in the sky on a moonless night. The sky is, however, seldom dark enough for the entire band of zodiacal light to be seen. Micrometeorites in interplanetary space, contributors to the zodiacal light, are constantly produced by asteroid collisions and liberated from the sublimating icy surfaces of comets. Since the radiation pressure of the sun is sufficient to blow submicron grains (beta meteoroids) out of the solar system, only larger grains (20–200 μm) contribute to the zodiacal light. Poynting-Robertson drag causes larger grains to depart from Keplerian orbits and to spiral slowly toward the sun.

Micrometeorites are the main contributors of extraterrestrial material accreted on the Earth. The accretion rate of cosmic dust particles has been estimated by various means so far, and the values calculated in those reports are different. There is, however, no difference in the conclusion that micrometeorites are the primary extraterrestrial deposit on Earth. Published reports estimating the accretion rate of extraterrestrial matter are well summarized in an appendix table of Peucker-Ehrenbrink (1996). For example, Love and Brownlee (1993) determined the mass flux and size distribution of micrometeoroids in the critical submillimeter size range by measuring hypervelocity impact craters found on the spacefacing end of the gravity-gradient-stabilized Long Duration Exposure Facility (LDEF) satellite. A small-particle mass accretion rate of 40,000 ± 20,000 tons/yr was obtained. In another estimate, a Japanese micrometeorite research group carefully picked up Antarctic micrometeorites and accurately counted their numbers, yielding accretion rates of 5,600– 10,400 tons/yr (Yada et al., 2001).

Although such a large amount of micrometeorites is continuously supplied to the Earth, micrometeorites have been collected in places where extraterrestrial particles are concentrated and/or terrestrial dust is rare, such as the deep sea, the stratosphere, and polar regions. It is very difficult to discover micrometeorites in inhabitable areas that are contaminated by artificial and terrestrial dusts. Since E. Nishibori collected micrometeorites in Antarctica in 1957–1958 (Nishibori and Ishizaki, 1959), a large number of micrometeorites have been recovered from the Antarctic and Greenland ice sheets and northern Canada

another versatile term synonymous with *cosmic dust*, but it is not as widely used as *cosmic* 

*Micrometeorite* is thought to be the best term representing extraterrestrial dust in this chapter for a few reasons. First, the cosmic dusts with solar wind noble gases reviewed here are not intergalactic dusts or interstellar dusts. Second, the Antarctic micrometeorites that are the main target of this paper are not IDPs. Therefore, the word *micrometeorite* adequately

It was already suspected in the Middle Ages that a large number of dusty objects existed in interplanetary space. Zodiacal light is a faint glow that extends away from the sun in the ecliptic plane of the sky, visible to the naked eye in the western sky shortly after sunset or in the eastern sky shortly before sunrise. Already in 1683, Giovanni Domenico Cassini presented the correct explanation of this prominent light phenomenon visible to the human eye. Its spectrum indicates it to be sunlight scattered by interplanetary dust orbiting the sun. It is called "counter-glow" or "Gegenschein" in German (Yamakoshi, 1994). The zodiacal light contributes about a third of the total light in the sky on a moonless night. The sky is, however, seldom dark enough for the entire band of zodiacal light to be seen. Micrometeorites in interplanetary space, contributors to the zodiacal light, are constantly produced by asteroid collisions and liberated from the sublimating icy surfaces of comets. Since the radiation pressure of the sun is sufficient to blow submicron grains (beta meteoroids) out of the solar system, only larger grains (20–200 μm) contribute to the zodiacal light. Poynting-Robertson drag causes larger grains to depart from Keplerian orbits

Micrometeorites are the main contributors of extraterrestrial material accreted on the Earth. The accretion rate of cosmic dust particles has been estimated by various means so far, and the values calculated in those reports are different. There is, however, no difference in the conclusion that micrometeorites are the primary extraterrestrial deposit on Earth. Published reports estimating the accretion rate of extraterrestrial matter are well summarized in an appendix table of Peucker-Ehrenbrink (1996). For example, Love and Brownlee (1993) determined the mass flux and size distribution of micrometeoroids in the critical submillimeter size range by measuring hypervelocity impact craters found on the spacefacing end of the gravity-gradient-stabilized Long Duration Exposure Facility (LDEF) satellite. A small-particle mass accretion rate of 40,000 ± 20,000 tons/yr was obtained. In another estimate, a Japanese micrometeorite research group carefully picked up Antarctic micrometeorites and accurately counted their numbers, yielding accretion rates of 5,600–

Although such a large amount of micrometeorites is continuously supplied to the Earth, micrometeorites have been collected in places where extraterrestrial particles are concentrated and/or terrestrial dust is rare, such as the deep sea, the stratosphere, and polar regions. It is very difficult to discover micrometeorites in inhabitable areas that are contaminated by artificial and terrestrial dusts. Since E. Nishibori collected micrometeorites in Antarctica in 1957–1958 (Nishibori and Ishizaki, 1959), a large number of micrometeorites have been recovered from the Antarctic and Greenland ice sheets and northern Canada

represents all types of cosmic dust that contain solar wind noble gases.

**1.2 Collection of micrometeorites** 

and to spiral slowly toward the sun.

10,400 tons/yr (Yada et al., 2001).

*dust*.

(Theil and Schmidt, 1961; Shima and Yabuki, 1968; Maurette et al., 1986, 1987, 1991; Koeberl and Hagen, 1989; Cresswell and Herd, 1992; Taylor et al., 1997, 1998; Nakamura et al., 1999; Yada and Kojima, 2000; Iwata and Imae, 2002; Rochette et al., 2008; Carole et al., 2011). Antarctic micrometeorites (AMMs) have larger sizes (50–300 μm) than the IDPs captured in the stratosphere (<30 μm). Since most of the mass accreted by the Earth is contained in larger particles (50–400 μm) (Kortenkamp and Dermott, 1998), AMMs represent the interplanetary dust population well.

## **2. Solar wind noble gases in deep-sea sediment**

Isotopic noble gas study on micrometeorites was difficult for a long time because of terrestrial contamination and the small sizes of micrometeorites. Measurements on single cosmic particles had to wait for great improvement of analytical devices. Therefore, the first noble gas study on micrometeorites was a measurement on deep-sea sediments in which micrometeorites were concentrated. The first noble gas isotopic study on deep-sea sediments was performed by Merrihue (1964). Magnetic and nonmagnetic separates of modern red clays from the Pacific Ocean were analyzed using a glass extraction and purification system, and excess 3He and 21Ne were discovered. The reported 3He/4He ratios (shown as 4He/3He in Merrihue's paper) are clearly higher than that of the terrestrial atmosphere, and a relatively high 20Ne/22Ne ratio (11.0 ± 1.0) is reported in the 1000°C step of the magnetic separate. 40Ar/36Ar ratios lower than that of the atmosphere in the 1000°C and 1400°C steps of the magnetic separate (268 ± 7 and 172 ± 8) were clearly detected, indicating the presence of extraterrestrial materials. This excellent research for the first time presented overwhelming evidence that extraterrestrial materials with extraterrestrial noble gases had accumulated in the deep-sea sediments. Nine years later, Krylov et al. (1973) reported He isotopic compositions of fifteen oceanic oozes recovered from various regions of the Pacific and Atlantic oceans and the iceberg-melting region of Greenland, which were analyzed by researchers in the Soviet Union. The isotopic ratios for Pacific red clays are tens or a hundred times that found in the various crustal rocks. On the other hand, Atlantic red clays have low 3He/4He ratios of 2–3 × 10−6 and no 3He anomaly was found in the Greenland samples. They believed that the likely source for the elevated 3He content in the Pacific Ocean sediments is cosmic rather than the hypothetical 3He from the mantle in the clays. The idea was confirmed by studying nitric-acid–treated ooze, which had the same order of 3He/4He ratios as untreated ooze. Indeed, the high 3He/4He ratios found in the red clays should be attributed to micrometeorites.

After these two reports, research in the field stagnated for a long time, and these important researches were forgotten completely. Japanese researchers, however, renewed study in the field in the 1980s. Ozima et al. (1984) measured thirty-nine sediments from twelve different sites, ten sites from the western to central Pacific and two sites from the Atlantic Ocean. They found 3He/4He ratios higher than 5 × 10−5 for six sites and concluded that the very high 3He/4He ratios in the sediments reflected the input of extraterrestrial materials. Amari and Ozima (1985) subsequently reported a He anomaly in deep-sea sediments, and they rediscovered that the carrier of exotic He was concentrated in magnetic fractions, which was consistent with the result of Merrihue's analysis. Since most terrestrial particles are nonmagnetic, magnetic cosmic dusts are concentrated in magnetic separates. They concluded that the ferromagnetic separates are essentially magnetite using thermomagnetic

Fig. 1. Reported 3He/4He ratios of deep-sea sediments. Dotted lines show the isotopic ratios of the terrestrial atmosphere at 1.4 × 10−6, implantation-fractionated solar wind (IFSW) at 2.17 × 10−4 (Benkert et al., 1993), and solar wind (SW) at 4.53 × 10−4 (Heber et al., 2008).

**3. Solar wind noble gases detected in individual unmelted micrometeorites**  Since noble gas isotope analysis for a single micrometeorite is very difficult because of the extremely small amount of noble gases in a particle, a mass spectrometer with high sensitivity and low background is required to determine accurate isotopic ratios of noble gases released from individual micrometeorites. The first attempt to measure single micrometeorites from deep Pacific Ocean sediments was made by Nier et al. (1987, 1990). They measured He and Ne in deep Pacific particles collected directly from the ocean floor with a 300 kg towed magnetic sled. The samples used were bulk magnetic fines that passed through a 100 μm sieve (they called them "deep Pacific magnetic fines") and individual particles larger than 100 μm in diameter. The individual particles were irregular, and their elemental composition, mineralogy, and texture were consistent with those of meteoritic materials. They measured thirty-five magnetic fines and six individual particles and suggested the possibility that there could be several types of extraterrestrial particles present in the magnetic fines. The most significant result in the paper was the extremely high He isotopic ratios observed in the 1600°C steps of the magnetic fines and individual particles.

They attributed the exotic noble gas compositions to solar flare particles.

IDPs collected from the stratosphere have provided valuable information on extraterrestrial noble gases trapped in cosmic dust particles. The first report concerning noble gas

analyses. They also performed a stepwise degassing experiment, which suggested that He is trapped fairly tightly. Amari and Ozima (1988) analyzed magnetic fractions separated from four deep-sea sediments from the Pacific Ocean. Notably, the study presented Ne and Ar isotopic compositions of the sediments. In all the samples, the 20Ne/22Ne ratios were constant (11.6 ± 0.6) in most temperature steps. This result should now be interpreted as being caused by a mixing of solar wind (SW) and implantation-fractionated solar wind (IFSW) components, although they concluded that the Ne was from a unique component. 40Ar/36Ar ratios lower than that of the atmosphere, 296, were evidently detected in hightemperature fractions of all samples, indicating the existence of extraterrestrial Ar. They concluded from the 20Ne/22Ne ratios and thermal release patterns of He that the extraterrestrial noble gases are implanted solar flare particles.

Fukumoto et al. (1986) determined elemental abundances and isotopic compositions of noble gases in separates and acid-leached residues of deep-sea sediments collected on a cruise of R/V Hakureimaru, Geological Survey of Japan. A 3He/4He ratio of (2.73 ± 0.06) × 10−4 was detected for the magnetic separate B2M. Nitric acid treatment did not affect the isotopic ratio, and the 3He/4He ratio of the leached sample B2M-1 is (2.74 ± 0.08) × 10−4, suggesting that the acid did not attack the carrier of the high 3He/4He ratio. Ne isotopic compositions show that the extraterrestrial materials in the sediments were affected by SW component rather than cosmic-ray spallation. Extraterrestrial Ar was detected in the acidleached residue B1-3, whose 40Ar/36Ar was 194.3 ± 52.2. Matsuda et al. (1990) carried out stepwise extraction analyses for the magnetic separate and 3M-HCl–leached residues of the same sample used by Fukumoto et al. (1986). Extraterrestrial He and Ne were observed in most temperature steps of all samples. The magnetic separate lost about 75% of its 3He without a drastic change in its isotopic ratios when it was dissolved in 3M HCl at room temperature for two days, and a sample more severely etched for six days had similar elemental and isotopic compositions of He and Ne to those of the two-day–etched sample, indicating that the extraterrestrial He and Ne should be concentrated in fine particles and/or on the surface of the magnetic grains. These studies performed by Japanese institutes clarified that extraterrestrial materials with solar-derived He and Ne are concentrated in deep-sea sediments and that the most plausible candidate for the carrier of the extraterrestrial noble gas is micrometeorites accreted on the Earth.

Reported 3He/4He ratios are summarized in Fig. 1. There are some differences in the isotopic ratios among the reports, and the ratio gradually increased with the year of the study, with the exception of the data from Merrihue (1964), reflecting the improvement in sample separation. Very high 3He/4He ratios were consistently detected in the study by Matsuda et al. (1990) because of their use of magnetic separation (they analyzed only 0.53% by weight of the dry sediment) and acid leaching. Such physical and chemical separations concentrated the extraterrestrial materials that exist in deep-sea sediment. The 3He/4He ratios reported by Merrihue (1964) are clearly too high, and the true values should be lower than the reported ratios. Ne isotopic compositions are also summarized in Fig. 2. The plots are distributed between the values of the SW and IFSW, indicating that the extraterrestrial materials in deep-sea sediments do not have chondritic noble gas compositions and that cosmogenic Ne is not dominant. The remarkably low 21Ne/22Ne ratios detected in the magnetic separates from deep-sea sediments are clearly consistent with the isotopic compositions of individual micrometeorites.

analyses. They also performed a stepwise degassing experiment, which suggested that He is trapped fairly tightly. Amari and Ozima (1988) analyzed magnetic fractions separated from four deep-sea sediments from the Pacific Ocean. Notably, the study presented Ne and Ar isotopic compositions of the sediments. In all the samples, the 20Ne/22Ne ratios were constant (11.6 ± 0.6) in most temperature steps. This result should now be interpreted as being caused by a mixing of solar wind (SW) and implantation-fractionated solar wind (IFSW) components, although they concluded that the Ne was from a unique component. 40Ar/36Ar ratios lower than that of the atmosphere, 296, were evidently detected in hightemperature fractions of all samples, indicating the existence of extraterrestrial Ar. They concluded from the 20Ne/22Ne ratios and thermal release patterns of He that the

Fukumoto et al. (1986) determined elemental abundances and isotopic compositions of noble gases in separates and acid-leached residues of deep-sea sediments collected on a cruise of R/V Hakureimaru, Geological Survey of Japan. A 3He/4He ratio of (2.73 ± 0.06) × 10−4 was detected for the magnetic separate B2M. Nitric acid treatment did not affect the isotopic ratio, and the 3He/4He ratio of the leached sample B2M-1 is (2.74 ± 0.08) × 10−4, suggesting that the acid did not attack the carrier of the high 3He/4He ratio. Ne isotopic compositions show that the extraterrestrial materials in the sediments were affected by SW component rather than cosmic-ray spallation. Extraterrestrial Ar was detected in the acidleached residue B1-3, whose 40Ar/36Ar was 194.3 ± 52.2. Matsuda et al. (1990) carried out stepwise extraction analyses for the magnetic separate and 3M-HCl–leached residues of the same sample used by Fukumoto et al. (1986). Extraterrestrial He and Ne were observed in most temperature steps of all samples. The magnetic separate lost about 75% of its 3He without a drastic change in its isotopic ratios when it was dissolved in 3M HCl at room temperature for two days, and a sample more severely etched for six days had similar elemental and isotopic compositions of He and Ne to those of the two-day–etched sample, indicating that the extraterrestrial He and Ne should be concentrated in fine particles and/or on the surface of the magnetic grains. These studies performed by Japanese institutes clarified that extraterrestrial materials with solar-derived He and Ne are concentrated in deep-sea sediments and that the most plausible candidate for the carrier of

Reported 3He/4He ratios are summarized in Fig. 1. There are some differences in the isotopic ratios among the reports, and the ratio gradually increased with the year of the study, with the exception of the data from Merrihue (1964), reflecting the improvement in sample separation. Very high 3He/4He ratios were consistently detected in the study by Matsuda et al. (1990) because of their use of magnetic separation (they analyzed only 0.53% by weight of the dry sediment) and acid leaching. Such physical and chemical separations concentrated the extraterrestrial materials that exist in deep-sea sediment. The 3He/4He ratios reported by Merrihue (1964) are clearly too high, and the true values should be lower than the reported ratios. Ne isotopic compositions are also summarized in Fig. 2. The plots are distributed between the values of the SW and IFSW, indicating that the extraterrestrial materials in deep-sea sediments do not have chondritic noble gas compositions and that cosmogenic Ne is not dominant. The remarkably low 21Ne/22Ne ratios detected in the magnetic separates from deep-sea sediments are clearly consistent with the isotopic

extraterrestrial noble gases are implanted solar flare particles.

the extraterrestrial noble gas is micrometeorites accreted on the Earth.

compositions of individual micrometeorites.

Fig. 1. Reported 3He/4He ratios of deep-sea sediments. Dotted lines show the isotopic ratios of the terrestrial atmosphere at 1.4 × 10−6, implantation-fractionated solar wind (IFSW) at 2.17 × 10−4 (Benkert et al., 1993), and solar wind (SW) at 4.53 × 10−4 (Heber et al., 2008).

## **3. Solar wind noble gases detected in individual unmelted micrometeorites**

Since noble gas isotope analysis for a single micrometeorite is very difficult because of the extremely small amount of noble gases in a particle, a mass spectrometer with high sensitivity and low background is required to determine accurate isotopic ratios of noble gases released from individual micrometeorites. The first attempt to measure single micrometeorites from deep Pacific Ocean sediments was made by Nier et al. (1987, 1990). They measured He and Ne in deep Pacific particles collected directly from the ocean floor with a 300 kg towed magnetic sled. The samples used were bulk magnetic fines that passed through a 100 μm sieve (they called them "deep Pacific magnetic fines") and individual particles larger than 100 μm in diameter. The individual particles were irregular, and their elemental composition, mineralogy, and texture were consistent with those of meteoritic materials. They measured thirty-five magnetic fines and six individual particles and suggested the possibility that there could be several types of extraterrestrial particles present in the magnetic fines. The most significant result in the paper was the extremely high He isotopic ratios observed in the 1600°C steps of the magnetic fines and individual particles. They attributed the exotic noble gas compositions to solar flare particles.

IDPs collected from the stratosphere have provided valuable information on extraterrestrial noble gases trapped in cosmic dust particles. The first report concerning noble gas

fragments had too little 4He to permit a determination. This result suggested that the parent IDPs of the twelve particles that contained an appreciable amount of 4He suffered very little heating in their descent and are likely of asteroidal origin, although one cannot rule out the possibility that at least some of them had a cometary origin and entered the Earth's atmosphere at a grazing angle. Nier and Schlutter later performed pulse-heating sequences for twenty-four individual IDPs to learn about the thermal history of the particles and distinguish between IDPs of asteroidal and cometary origin. In this investigation, fifteen of twenty-four particles had 3He/4He ratios above 10−3, and the highest value, 2 × 10−2, was

Kehm et al. (1998a) performed combined trace element and light noble gas measurements on fourteen IDPs from the L2036 stratospheric collector using a laser gas-extraction system and a synchrotron X-ray microprobe. The Ne isotopic compositions in these IDPs were dominated by implanted solar components including SW and IFSW Ne. The Ar isotopic compositions of six large IDPs (>25 μm in their longest dimension) demonstrated enrichment in solar components. Low 4He contents were observed in five particles that exhibited Zn depletion, indicating severe heating and volatile loss during atmospheric entry. Kehm et al. (1998b) later performed trace element and noble gas measurements on ten large IDPs (~20 μm). They suggested preferential He loss during atmospheric entry heating in this study. Kehm et al. (1999) performed noble gas measurements on JJ-91 IDPs and presented major differences between the result of their measurements and the data of Nier and Schlutter (1993). Kehm et al. (1999) did not detect an anomalously high 3He/4He ratio in a fragment of 2011 cluster 11, in which a very high 3He/4He ratio was detected by Nier and Schlutter (1993). However, the reasons for the differences were not clear. Kehm et al. (2002) measured noble gases in 32 individual IDPs, and the 4He, 20Ne, and 36Ar contents were determined for 31 IDPs. The noble gas elemental compositions were consistent with the

presence of fractionated solar wind, but the isotopic compositions were unknown.

although their noble gas concentrations were very low due to severe heating.

Ne isotopic compositions of individual unmelted micrometeorites collected from seasonal lakes on the Greenland ice sheet were reported by Olinger et al. (1990). The extraterrestrial origin of the particles was confirmed by the isotopic data. Maurette et al. (1991) reported Ne isotopic compositions of unmelted and partially melted micrometeorites recovered from Antarctic blue ice. Stuart et al. (1999) measured He isotopes in forty-five putative micrometeorites in the size range of 50–400 μm recovered from Antarctic ice. They determined the He isotopic compositions of twenty-six particles. Pepin et al. (2000, 2001) reported He, Ne, and Ar isotopic ratios for many IDPs and discussed the extremely high 3He concentration found in some large cluster particles by Nier and Schlutter (1993). They proposed several possibilities to explain the overabundance of 3He. The noble gas research group at the University of Tokyo reported isotopic compositions of noble gases including Ar, Kr, and Xe for individual unmelted AMMs using a highly established mass spectrometer with a laser gas extraction system (Osawa and Nagao, 2002a, 2002b; Osawa et al., 2000, 2001, 2003). These studies clarified that many micrometeorites contain not only extraterrestrial He and Ne but also extraterrestrial Ar. It is, however, very difficult to detect extraterrestrial Kr and Xe because the concentrations of heavy noble gases are extremely low and the effect of adsorbed terrestrial atmosphere cannot be ignored. Osawa and Nagao (2003) and Osawa et al. (2010) reported noble gas compositions of individual cosmic spherules recovered from Antarctica, and about 40% of the cosmic spherules preserved extraterrestrial noble gases,

found in L2011 D7. They had no explanation for this anomaly.

Fig. 2. Three-isotope plot of Ne for deep-sea sediments. SW and IFSW data are from Heber et al. (2008) and Benkert et al. (1993), respectively.

compositions of IDPs is that by Rajan et al. (1977). They detected very high concentrations of 4He ranging from 0.002 to 0.25 cm3 STP/g in ten stratospheric particles collected by NASA U-2 aircraft and asserted that the particles were extraterrestrial and that some or all of them were exposed to solar wind for at least 10–100 years. Hudson et al. (1981) selected thirteen chondritic stratospheric particles and measured Ne, Ar, Kr, and Xe by stepwise heating at 1400°C, 1500°C, and 1600°C. The 20Ne/36Ar ratio in the particles is 9 ± 3, indicating the presence of solar-type light noble gas. On the other hand, the 132Xe concentration of ~10−<sup>7</sup> cm3 STP/g and the heavy noble gas elemental pattern suggested a substantial contribution from planetary sources. This is the only report on Kr and Xe in extraterrestrial dusts before Osawa et al. (2000).

The first noble gas measurement for individual IDPs was performed by Nier and Schlutter (1989). They measured He and Ne isotopic compositions for sixteen individual stratospheric particles. The samples were wrapped in a small piece of previously degassed Ta foil, and noble gases were extracted by heating, which was accomplished by passing an electric current directly through the foil. Except for one sample, the IDPs had 3He/4He ratios of 1.5– 4.3 × 10−4. The average of the 20Ne/22Ne ratio was 12.0 ± 0.5. In the next stage, they performed stepwise heating for fragments from twenty individual particles to clarify the origin of the particles using the release pattern of 4He (Nier and Schlutter, 1992). Twelve of the IDP fragments contained an appreciable amount of 4He, 50% of which was released by the time the particles were heated to approximately 630°C. Four IDP fragments contained appreciably less 4He, and this was released at a higher temperature. The remaining four

Fig. 2. Three-isotope plot of Ne for deep-sea sediments. SW and IFSW data are from Heber

compositions of IDPs is that by Rajan et al. (1977). They detected very high concentrations of 4He ranging from 0.002 to 0.25 cm3 STP/g in ten stratospheric particles collected by NASA U-2 aircraft and asserted that the particles were extraterrestrial and that some or all of them were exposed to solar wind for at least 10–100 years. Hudson et al. (1981) selected thirteen chondritic stratospheric particles and measured Ne, Ar, Kr, and Xe by stepwise heating at 1400°C, 1500°C, and 1600°C. The 20Ne/36Ar ratio in the particles is 9 ± 3, indicating the presence of solar-type light noble gas. On the other hand, the 132Xe concentration of ~10−<sup>7</sup> cm3 STP/g and the heavy noble gas elemental pattern suggested a substantial contribution from planetary sources. This is the only report on Kr and Xe in extraterrestrial dusts before

The first noble gas measurement for individual IDPs was performed by Nier and Schlutter (1989). They measured He and Ne isotopic compositions for sixteen individual stratospheric particles. The samples were wrapped in a small piece of previously degassed Ta foil, and noble gases were extracted by heating, which was accomplished by passing an electric current directly through the foil. Except for one sample, the IDPs had 3He/4He ratios of 1.5– 4.3 × 10−4. The average of the 20Ne/22Ne ratio was 12.0 ± 0.5. In the next stage, they performed stepwise heating for fragments from twenty individual particles to clarify the origin of the particles using the release pattern of 4He (Nier and Schlutter, 1992). Twelve of the IDP fragments contained an appreciable amount of 4He, 50% of which was released by the time the particles were heated to approximately 630°C. Four IDP fragments contained appreciably less 4He, and this was released at a higher temperature. The remaining four

et al. (2008) and Benkert et al. (1993), respectively.

Osawa et al. (2000).

fragments had too little 4He to permit a determination. This result suggested that the parent IDPs of the twelve particles that contained an appreciable amount of 4He suffered very little heating in their descent and are likely of asteroidal origin, although one cannot rule out the possibility that at least some of them had a cometary origin and entered the Earth's atmosphere at a grazing angle. Nier and Schlutter later performed pulse-heating sequences for twenty-four individual IDPs to learn about the thermal history of the particles and distinguish between IDPs of asteroidal and cometary origin. In this investigation, fifteen of twenty-four particles had 3He/4He ratios above 10−3, and the highest value, 2 × 10−2, was found in L2011 D7. They had no explanation for this anomaly.

Kehm et al. (1998a) performed combined trace element and light noble gas measurements on fourteen IDPs from the L2036 stratospheric collector using a laser gas-extraction system and a synchrotron X-ray microprobe. The Ne isotopic compositions in these IDPs were dominated by implanted solar components including SW and IFSW Ne. The Ar isotopic compositions of six large IDPs (>25 μm in their longest dimension) demonstrated enrichment in solar components. Low 4He contents were observed in five particles that exhibited Zn depletion, indicating severe heating and volatile loss during atmospheric entry. Kehm et al. (1998b) later performed trace element and noble gas measurements on ten large IDPs (~20 μm). They suggested preferential He loss during atmospheric entry heating in this study. Kehm et al. (1999) performed noble gas measurements on JJ-91 IDPs and presented major differences between the result of their measurements and the data of Nier and Schlutter (1993). Kehm et al. (1999) did not detect an anomalously high 3He/4He ratio in a fragment of 2011 cluster 11, in which a very high 3He/4He ratio was detected by Nier and Schlutter (1993). However, the reasons for the differences were not clear. Kehm et al. (2002) measured noble gases in 32 individual IDPs, and the 4He, 20Ne, and 36Ar contents were determined for 31 IDPs. The noble gas elemental compositions were consistent with the presence of fractionated solar wind, but the isotopic compositions were unknown.

Ne isotopic compositions of individual unmelted micrometeorites collected from seasonal lakes on the Greenland ice sheet were reported by Olinger et al. (1990). The extraterrestrial origin of the particles was confirmed by the isotopic data. Maurette et al. (1991) reported Ne isotopic compositions of unmelted and partially melted micrometeorites recovered from Antarctic blue ice. Stuart et al. (1999) measured He isotopes in forty-five putative micrometeorites in the size range of 50–400 μm recovered from Antarctic ice. They determined the He isotopic compositions of twenty-six particles. Pepin et al. (2000, 2001) reported He, Ne, and Ar isotopic ratios for many IDPs and discussed the extremely high 3He concentration found in some large cluster particles by Nier and Schlutter (1993). They proposed several possibilities to explain the overabundance of 3He. The noble gas research group at the University of Tokyo reported isotopic compositions of noble gases including Ar, Kr, and Xe for individual unmelted AMMs using a highly established mass spectrometer with a laser gas extraction system (Osawa and Nagao, 2002a, 2002b; Osawa et al., 2000, 2001, 2003). These studies clarified that many micrometeorites contain not only extraterrestrial He and Ne but also extraterrestrial Ar. It is, however, very difficult to detect extraterrestrial Kr and Xe because the concentrations of heavy noble gases are extremely low and the effect of adsorbed terrestrial atmosphere cannot be ignored. Osawa and Nagao (2003) and Osawa et al. (2010) reported noble gas compositions of individual cosmic spherules recovered from Antarctica, and about 40% of the cosmic spherules preserved extraterrestrial noble gases, although their noble gas concentrations were very low due to severe heating.

their grain sizes (Stuart et al., 1999). IDPs are smaller than AMMs and have a higher surface area/volume ratio than do AMMs. Since the mechanism of accumulation of SW noble gases in micrometeorites is ion implantation, the concentration of SW noble gases depends on surface area. A high surface area/volume ratio thus causes a high noble gas concentration. A secondary reason for the high He concentration of IDPs is the lower heating temperature; IDPs can escape severe heating because of their low weight and density. He loss in AMMs occurs in response to the thermal decomposition of phyllosilicates and diffusive loss and bubble rupture during atmospheric entry, rather than melting (Stuart et al., 1999). Aqueous alteration in the Antarctic snow can be another possible cause of He loss in AMMs. For example, jarosite [KFe3(SO4)2(OH)6], a by-product mineral resulting from aqueous alteration of sulfide minerals, is observed in ~43% of the AMMs collected from 30,000 year old glacial ice (Terada et al., 2001), and these AMMs have lower He concentrations than AMMs collected from fresh snow, indicating He loss due to aqueous alteration (Osawa and Nagao, 2002). Osawa et al. (2003) reported that jarosite-bearing AMMs have relatively low concentrations of 4He, suggesting loss of He during long-term storage in ice. However, since jarosite is not often found in AMMs, aqueous alteration in ice is not the main cause of the

Although the He isotopic ratios of most AMMs and IDPs simply reflect solar-derived He, it is not possible to completely deny the contributions of other components such as planetary He and cosmogenic 3He, an additional component found in some IDPs. In addition, isotopic fractionation during entry deceleration heating should be taken into consideration. Some AMMs and IDPs have higher 3He/4He ratios than that of SW. These probably reflect cosmogenic 3He because the 3He/4He ratio of cosmogenic He is very high, about 0.2. Since cosmogenic 3He is more strongly retained in a micrometeorite than SW He, which exists mostly in the surface layer because of the low energy of solar wind, the 3He/4He ratio is elevated by the preferential loss of solar-wind–derived He. If cosmogenic 3He does not exist in the AMMs, the 3He/4He ratio will approach the ratio of IFSW after the loss of the surface layer of the micrometeorites (Grimberg et al., 2008). The cosmogenic 3He concentrations of some unmelted AMMs with relatively high 3He/4He ratios are much lower than those of IDPs with high concentrations of cosmogenic 3He of over 5 × 10−6 cm3 STP/g (Pepin et al., 2001). Strikingly high 3He/4He ratios, possibly due to some unknown reservoir, were reported for some IDPs (Nier and Schlutter, 1993; Pepin et al., 2000). For example, the IDP L2011D7 has a low 4He content (3.4 × 10−12 cm3STP) and an unusually high 3He/4He ratio ((2.0 ± 0.3) × 10−2; Nier and Schlutter, 1993). Kehm et al. (1999), however, did not detect such anomalously high 3He/4He ratios in individual IDP grains separated from the same cluster IDP L2011. In their measurement, nine of eleven IDPs had high He content (0.7–7 × 10−10 cm3STP) and low 3He/4He ratios; the He compositions correspond to those of typical IDPs shown in Fig. 3. The high 3He/4He ratios found in the enigmatic IDPs are thus very problematic. If the large overabundance of 3He is to be attributed to cosmogenic 3He, extremely long periods of cosmic-ray irradiation time are required (Pepin et al., 2001). It is noted that the lack of Ne isotopic data obstructs the interpretation of the problem of excess 3He in IDPs. Even if the enigmatic IDPs are excluded in this discussion, the excess 3He concentrations of AMMs are clearly low compared to those of IDPs. Since the low concentration of cosmogenic 3He presumably indicates preferential loss of He due to severe entry heating, 3He exposure

low He concentration of AMMs.

ages of AMMs are not reliable, in contrast to those of IDPs.

#### **3.1 He isotopic ratios of micrometeorites**

Compiled He isotope data for unmelted AMMs and IDPs are depicted in Fig. 3. The data on IDPs with strikingly high 3He/4He ratios reported by Nier and Schlutter (1993) are excluded here. The 3He/4He ratios in the AMMs and IDPs are plotted against the concentrations of 4He in this figure. The range of 4He concentrations extends from 10−6 to 10 cm3 STP/g, which may reflect the degree of entry heating for each AMM and IDP. The 3He/4He ratios of most AMMs are distributed between those of SW and IFSW value, showing the presence of SW He, but there is no significant correlation between the isotopic ratios and 4He concentration. Since the SW noble gas is thought to become saturated in the surface layer of a small particle in interplanetary space within about a few decades (e.g., Hudson et al., 1981), solar-wind–derived He is implanted in the surface of AMMs and IDPs. It is, however, notable that the isotopic ratios are not clustered around the SW value, and more than half of the particles have 3He/4He ratios lower than that of SW. This is due to isotopic fractionation during solar wind ion implantation and the loss of the surface layer of the particles during atmospheric entry. The surface layers of the micrometeorites were preferentially heated and ablated by flash heating (e.g., Love and Brownlee, 1991). However, the SW He in the micrometeorites had not been completely extracted by the heating, and the remaining solarwind–derived He proves the extraterrestrial origin of the AMMs and IDPs.

Fig. 3. 4He concentration and 3He/4He ratio of unmelted AMMs and IDPs. IDP data are from Nier and Schlutter (1990, 1992) and Pepin et al. (2000, 2011). Unmelted AMM data are from Stuart et al. (1999), Osawa and Nagao (2002b), and Osawa et al. (2003).

The very large difference in 4He concentration between AMMs and IDPs is remarkable; IDPs have a much higher concentration of 4He than do AMMs, but the 3He/4He ratio of most IDPs falls in a similar range to that of AMMs. The large difference in 4He concentration is mainly caused by the size range; 4He concentrations in cosmic dust particles correlate with

Compiled He isotope data for unmelted AMMs and IDPs are depicted in Fig. 3. The data on IDPs with strikingly high 3He/4He ratios reported by Nier and Schlutter (1993) are excluded here. The 3He/4He ratios in the AMMs and IDPs are plotted against the concentrations of 4He in this figure. The range of 4He concentrations extends from 10−6 to 10 cm3 STP/g, which may reflect the degree of entry heating for each AMM and IDP. The 3He/4He ratios of most AMMs are distributed between those of SW and IFSW value, showing the presence of SW He, but there is no significant correlation between the isotopic ratios and 4He concentration. Since the SW noble gas is thought to become saturated in the surface layer of a small particle in interplanetary space within about a few decades (e.g., Hudson et al., 1981), solar-wind–derived He is implanted in the surface of AMMs and IDPs. It is, however, notable that the isotopic ratios are not clustered around the SW value, and more than half of the particles have 3He/4He ratios lower than that of SW. This is due to isotopic fractionation during solar wind ion implantation and the loss of the surface layer of the particles during atmospheric entry. The surface layers of the micrometeorites were preferentially heated and ablated by flash heating (e.g., Love and Brownlee, 1991). However, the SW He in the micrometeorites had not been completely extracted by the heating, and the remaining solar-

wind–derived He proves the extraterrestrial origin of the AMMs and IDPs.

Fig. 3. 4He concentration and 3He/4He ratio of unmelted AMMs and IDPs. IDP data are from Nier and Schlutter (1990, 1992) and Pepin et al. (2000, 2011). Unmelted AMM data are

The very large difference in 4He concentration between AMMs and IDPs is remarkable; IDPs have a much higher concentration of 4He than do AMMs, but the 3He/4He ratio of most IDPs falls in a similar range to that of AMMs. The large difference in 4He concentration is mainly caused by the size range; 4He concentrations in cosmic dust particles correlate with

from Stuart et al. (1999), Osawa and Nagao (2002b), and Osawa et al. (2003).

**3.1 He isotopic ratios of micrometeorites** 

their grain sizes (Stuart et al., 1999). IDPs are smaller than AMMs and have a higher surface area/volume ratio than do AMMs. Since the mechanism of accumulation of SW noble gases in micrometeorites is ion implantation, the concentration of SW noble gases depends on surface area. A high surface area/volume ratio thus causes a high noble gas concentration. A secondary reason for the high He concentration of IDPs is the lower heating temperature; IDPs can escape severe heating because of their low weight and density. He loss in AMMs occurs in response to the thermal decomposition of phyllosilicates and diffusive loss and bubble rupture during atmospheric entry, rather than melting (Stuart et al., 1999). Aqueous alteration in the Antarctic snow can be another possible cause of He loss in AMMs. For example, jarosite [KFe3(SO4)2(OH)6], a by-product mineral resulting from aqueous alteration of sulfide minerals, is observed in ~43% of the AMMs collected from 30,000 year old glacial ice (Terada et al., 2001), and these AMMs have lower He concentrations than AMMs collected from fresh snow, indicating He loss due to aqueous alteration (Osawa and Nagao, 2002). Osawa et al. (2003) reported that jarosite-bearing AMMs have relatively low concentrations of 4He, suggesting loss of He during long-term storage in ice. However, since jarosite is not often found in AMMs, aqueous alteration in ice is not the main cause of the low He concentration of AMMs.

Although the He isotopic ratios of most AMMs and IDPs simply reflect solar-derived He, it is not possible to completely deny the contributions of other components such as planetary He and cosmogenic 3He, an additional component found in some IDPs. In addition, isotopic fractionation during entry deceleration heating should be taken into consideration. Some AMMs and IDPs have higher 3He/4He ratios than that of SW. These probably reflect cosmogenic 3He because the 3He/4He ratio of cosmogenic He is very high, about 0.2. Since cosmogenic 3He is more strongly retained in a micrometeorite than SW He, which exists mostly in the surface layer because of the low energy of solar wind, the 3He/4He ratio is elevated by the preferential loss of solar-wind–derived He. If cosmogenic 3He does not exist in the AMMs, the 3He/4He ratio will approach the ratio of IFSW after the loss of the surface layer of the micrometeorites (Grimberg et al., 2008). The cosmogenic 3He concentrations of some unmelted AMMs with relatively high 3He/4He ratios are much lower than those of IDPs with high concentrations of cosmogenic 3He of over 5 × 10−6 cm3 STP/g (Pepin et al., 2001). Strikingly high 3He/4He ratios, possibly due to some unknown reservoir, were reported for some IDPs (Nier and Schlutter, 1993; Pepin et al., 2000). For example, the IDP L2011D7 has a low 4He content (3.4 × 10−12 cm3STP) and an unusually high 3He/4He ratio ((2.0 ± 0.3) × 10−2; Nier and Schlutter, 1993). Kehm et al. (1999), however, did not detect such anomalously high 3He/4He ratios in individual IDP grains separated from the same cluster IDP L2011. In their measurement, nine of eleven IDPs had high He content (0.7–7 × 10−10 cm3STP) and low 3He/4He ratios; the He compositions correspond to those of typical IDPs shown in Fig. 3. The high 3He/4He ratios found in the enigmatic IDPs are thus very problematic. If the large overabundance of 3He is to be attributed to cosmogenic 3He, extremely long periods of cosmic-ray irradiation time are required (Pepin et al., 2001). It is noted that the lack of Ne isotopic data obstructs the interpretation of the problem of excess 3He in IDPs. Even if the enigmatic IDPs are excluded in this discussion, the excess 3He concentrations of AMMs are clearly low compared to those of IDPs. Since the low concentration of cosmogenic 3He presumably indicates preferential loss of He due to severe entry heating, 3He exposure ages of AMMs are not reliable, in contrast to those of IDPs.

spite of their severe heating. This result implies that the spherules are small particles in interplanetary space and not fragments of meteorites fallen to the Earth, as solar-gas–rich meteorites are quite rare. Osawa et al. (2010) discovered an exotic cosmic spherule, M240410, which has an extraordinarily high 3He/4He ratio ((9.7 ± 1.1) × 10−3) and high 3He content (5.53 × 10−13 cm3STP) that resulted from cosmogenic production of 3He. Such high isotopic ratios have not been found in unmelted micrometeorites, indicating that this specific spherule may have an exceptional history. The highest 3He/4He ratio reported to date in an unmelted micrometeorite is (1.843 ± 0.050) × 10−3 (Stuart et al., 1999), which is much lower

Ne isotope data on micrometeorites can provide information on solar wind, fractionated solar wind, and cosmogenic nuclides. These three components can be separated using a diagram because Ne has three stable isotopes in contrast with He, which has only two. The Ne isotopic composition is thus useful for separating SW components from cosmogenic nuclides, but the Ne concentration of micrometeorites is much lower than the He

Fig. 5 displays Ne isotopic compositions of unmelted micrometeorites, IDPs, and cosmic spherules. It is remarkable that most micrometeorite data are clustered around the IFSW value and show no cosmogenic 21Ne within the error limit, indicating short exposure ages. Several micrometeorites have 21Ne/22Ne ratios higher than that of SW; for example, two exceptional Dome Fuji AMMs have long cosmic-ray exposure (CRE) ages (>100 Myr). However, most micrometeorites have exposure ages shorter than 1 Myr (Osawa and Nagao, 2002a). An enigmatic cosmic spherule, M240410, has an extremely high concentration of cosmogenic 21Ne and was calculated to have a very long CRE age of 393 Myr when 4π exposure to galactic and solar cosmic rays was taken into consideration, indicating that the source of the particle may have been an Edgeworth-Kuiper belt object (Osawa et al., 2010). The Ne isotopic compositions of several unmelted micrometeorites are close to, or above, the SW 20Ne/22Ne ratio of 13.77 (Heber et al., 2008). These are Greenland micrometeorite compositions reported by Olinger et al. (1990), and the high 20Ne/22Ne ratios are due to the overestimation of CO2++ interference. Hence, the SW-like Ne compositions detected in some micrometeorites do not indicate the presence of unfractionated solar wind, and the solar-

derived Ne in all types of micrometeorites is partially depleted and fractionated.

simple atmospheric entry ablation fragments of meteorites.

The effect of partial loss of Ne can be observed in a trend in the 20Ne/22Ne ratio. The average 20Ne/22Ne ratios of IDPs, unmelted micrometeorites, and cosmic spherules are 11.92, 11.39, and 10.57, respectively; the difference in the isotopic ratios among the three micrometeorite groups may reflect the degree of atmospheric entry heating. The smaller IDPs (~20 μm) experienced lower entry temperatures compared to the larger micrometeorites (~100 μm) because the maximum temperature during the trajectory depends on particle radius (e.g., Rizk et al., 1991). The average 20Ne/22Ne ratio of cosmic spherules is lower than the IFSW ratio, 11.3, reflecting contamination by the terrestrial atmosphere. Although noble gases in cosmic spherules are considerably depleted by severe flash heating, some spherules preserved solar-wind–derived He and Ne, suggesting that the cosmic spherules have been exposed to solar wind and/or solar flares before atmospheric entry and that they are not

than that of M240410.

concentration.

**3.2 Ne isotopic ratios of micrometeorites** 

The geometric average of 3He/4He ratios of AMMs, 3.10 × 10−4, is slightly lower than that of IDPs, 3.55 × 10−4, which may also reflect the difference in the degree of surface loss or heating during atmospheric entry. This result is consistent with the large difference in He concentration between the two micrometeorite series. Note that a geometric average is more suitable for evaluating the representative He isotopic ratio of micrometeorite samples than an arithmetic mean because the distributions of 3He/4He ratios of AMMs and IDPs are evidently not normal distributions. In conclusion, unmelted AMMs and IDPs preserve extraterrestrial He derived from energetic implantation of solar wind, but the effects of gas loss and fractionation cannot be ignored. SW He trapped in micrometeorites found on the Earth does not, therefore, represent pure solar wind.

It is extremely difficult to detect solar wind noble gases in the totally melted cosmic spherules because most volatiles have been depleted by harsh heating during atmospheric entry. It is, however, surprising that extraterrestrial He, Ne, and Ar still remain in some cosmic spherules (Osawa and Nagao, 2003; Osawa et al., 2010). Fig. 4 shows the 4He contents and the 3He/4He ratios of unmelted AMMs and cosmic spherules. Since only 29 of 130 spherules preserved detectable amounts of 3He, the 4He contents of the spherules presented in Fig. 4 do not reflect the distribution of the noble gas contents of all spherules. Even the 4He contents of the gas-rich cosmic spherules shown in the figure are much lower than those of unmelted AMMs. All of the gas-rich cosmic spherules have 3He/4He ratios higher than that of terrestrial air within one sigma error, proving their extraterrestrial origin. Furthermore, many spherules have He isotopic ratios close to that of SW, as do the unmelted micrometeorites, indicating that the spherules have preserved solar-derived He in

Fig. 4. Relationship between 4He content and 3He/4He ratio of unmelted AMMs and cosmic spherules. Unmelted AMM data are from Stuart et al. (1999), Osawa and Nagao (2002b), and Osawa et al. (2003). Cosmic spherule data are from Osawa and Nagao (2003) and Osawa et al. (2010).

The geometric average of 3He/4He ratios of AMMs, 3.10 × 10−4, is slightly lower than that of IDPs, 3.55 × 10−4, which may also reflect the difference in the degree of surface loss or heating during atmospheric entry. This result is consistent with the large difference in He concentration between the two micrometeorite series. Note that a geometric average is more suitable for evaluating the representative He isotopic ratio of micrometeorite samples than an arithmetic mean because the distributions of 3He/4He ratios of AMMs and IDPs are evidently not normal distributions. In conclusion, unmelted AMMs and IDPs preserve extraterrestrial He derived from energetic implantation of solar wind, but the effects of gas loss and fractionation cannot be ignored. SW He trapped in micrometeorites found on the

It is extremely difficult to detect solar wind noble gases in the totally melted cosmic spherules because most volatiles have been depleted by harsh heating during atmospheric entry. It is, however, surprising that extraterrestrial He, Ne, and Ar still remain in some cosmic spherules (Osawa and Nagao, 2003; Osawa et al., 2010). Fig. 4 shows the 4He contents and the 3He/4He ratios of unmelted AMMs and cosmic spherules. Since only 29 of 130 spherules preserved detectable amounts of 3He, the 4He contents of the spherules presented in Fig. 4 do not reflect the distribution of the noble gas contents of all spherules. Even the 4He contents of the gas-rich cosmic spherules shown in the figure are much lower than those of unmelted AMMs. All of the gas-rich cosmic spherules have 3He/4He ratios higher than that of terrestrial air within one sigma error, proving their extraterrestrial origin. Furthermore, many spherules have He isotopic ratios close to that of SW, as do the unmelted micrometeorites, indicating that the spherules have preserved solar-derived He in

Fig. 4. Relationship between 4He content and 3He/4He ratio of unmelted AMMs and cosmic spherules. Unmelted AMM data are from Stuart et al. (1999), Osawa and Nagao (2002b), and Osawa et al. (2003). Cosmic spherule data are from Osawa and Nagao (2003) and Osawa et

Earth does not, therefore, represent pure solar wind.

al. (2010).

spite of their severe heating. This result implies that the spherules are small particles in interplanetary space and not fragments of meteorites fallen to the Earth, as solar-gas–rich meteorites are quite rare. Osawa et al. (2010) discovered an exotic cosmic spherule, M240410, which has an extraordinarily high 3He/4He ratio ((9.7 ± 1.1) × 10−3) and high 3He content (5.53 × 10−13 cm3STP) that resulted from cosmogenic production of 3He. Such high isotopic ratios have not been found in unmelted micrometeorites, indicating that this specific spherule may have an exceptional history. The highest 3He/4He ratio reported to date in an unmelted micrometeorite is (1.843 ± 0.050) × 10−3 (Stuart et al., 1999), which is much lower than that of M240410.

#### **3.2 Ne isotopic ratios of micrometeorites**

Ne isotope data on micrometeorites can provide information on solar wind, fractionated solar wind, and cosmogenic nuclides. These three components can be separated using a diagram because Ne has three stable isotopes in contrast with He, which has only two. The Ne isotopic composition is thus useful for separating SW components from cosmogenic nuclides, but the Ne concentration of micrometeorites is much lower than the He concentration.

Fig. 5 displays Ne isotopic compositions of unmelted micrometeorites, IDPs, and cosmic spherules. It is remarkable that most micrometeorite data are clustered around the IFSW value and show no cosmogenic 21Ne within the error limit, indicating short exposure ages. Several micrometeorites have 21Ne/22Ne ratios higher than that of SW; for example, two exceptional Dome Fuji AMMs have long cosmic-ray exposure (CRE) ages (>100 Myr). However, most micrometeorites have exposure ages shorter than 1 Myr (Osawa and Nagao, 2002a). An enigmatic cosmic spherule, M240410, has an extremely high concentration of cosmogenic 21Ne and was calculated to have a very long CRE age of 393 Myr when 4π exposure to galactic and solar cosmic rays was taken into consideration, indicating that the source of the particle may have been an Edgeworth-Kuiper belt object (Osawa et al., 2010). The Ne isotopic compositions of several unmelted micrometeorites are close to, or above, the SW 20Ne/22Ne ratio of 13.77 (Heber et al., 2008). These are Greenland micrometeorite compositions reported by Olinger et al. (1990), and the high 20Ne/22Ne ratios are due to the overestimation of CO2++ interference. Hence, the SW-like Ne compositions detected in some micrometeorites do not indicate the presence of unfractionated solar wind, and the solarderived Ne in all types of micrometeorites is partially depleted and fractionated.

The effect of partial loss of Ne can be observed in a trend in the 20Ne/22Ne ratio. The average 20Ne/22Ne ratios of IDPs, unmelted micrometeorites, and cosmic spherules are 11.92, 11.39, and 10.57, respectively; the difference in the isotopic ratios among the three micrometeorite groups may reflect the degree of atmospheric entry heating. The smaller IDPs (~20 μm) experienced lower entry temperatures compared to the larger micrometeorites (~100 μm) because the maximum temperature during the trajectory depends on particle radius (e.g., Rizk et al., 1991). The average 20Ne/22Ne ratio of cosmic spherules is lower than the IFSW ratio, 11.3, reflecting contamination by the terrestrial atmosphere. Although noble gases in cosmic spherules are considerably depleted by severe flash heating, some spherules preserved solar-wind–derived He and Ne, suggesting that the cosmic spherules have been exposed to solar wind and/or solar flares before atmospheric entry and that they are not simple atmospheric entry ablation fragments of meteorites.

Fig. 6. Ar isotopic compositions of (a) unmelted AMMs and (b) cosmic spherules. Unmelted AMM data are from Osawa and Nagao (2002b) and Osawa et al. (2003). Cosmic spherule

Cosmic-ray–produced spallogenic 38Ar was detected only in spherule M240410, which has a detectable amount of cosmogenic 3He and 21Ne. The concentration of cosmogenic 38Ar of the spherule is 4.8 × 10−8 cm3 STP/g, and the CRE age calculated with 2π irradiation is 382.1 Myr (Osawa et al., 2010). All micrometeorites other than this exceptional spherule have no cosmogenic 38Ar, even the unmelted AMMs with relatively high 21Ne/22Ne ratios, presumably due to the lower rate of cosmic-ray production of 38Ar than that of 21Ne

data are from Osawa and Nagao (2003) and Osawa et al. (2010).

(Eugster, 1988).

Fig. 5. Three-isotope plot of Ne for unmelted AMMs, cosmic spherules, and IDPs. Unmelted AMM data are from Olinger et al. (1990), Osawa and Nagao (2002b), and Osawa et al. (2003). Cosmic spherule data are from Osawa and Nagao (2003) and Osawa et al. (2010). IDP data are from Pepin et al. (2000). An arrow shows the direction of cosmogenic Ne.

#### **3.3 Ar isotopic ratios of micrometeorites**

Ar isotopic compositions of individual micrometeorites were reported only by two groups, at Washington University and the University of Tokyo (Kehm et al., 1998a; Osawa and Nagao, 2002a, 2002b, 2003; Osawa et al., 2000, 2001, 2003, 2010). Merrihue (1964) reported a low 40Ar/36Ar ratio (172 ± 5 in the 1400°C fraction) in a magnetic separate of Pacific red clay and suggested that it contains meteoritic material, but that the data do not correspond to those of a single micrometeorite. Since Ar has three stable isotopes, as does Ne, the Ar isotopic compositions of micrometeorites can clarify the contributions of more than two components. A three-isotope plot of Ar for unmelted AMMs and cosmic spherules is presented in Fig. 6. IDP data from Kehm et al. (1998a) are not plotted in this diagram because of the lack of raw data. All unmelted micrometeorites with detectable amounts of Ar have 40Ar/36Ar ratios lower than that of the terrestrial atmosphere, 296, confirming their classification as extraterrestrial because terrestrial materials with 40Ar/36Ar ratios lower than that of terrestrial air are very few. Although the Ar isotopic compositions of cosmic spherules have large uncertainties due to the very low Ar concentrations, the 40Ar/36Ar ratios of many spherules are lower than the atmospheric value. This indicates that extraterrestrial Ar is detectable for these samples because significant gas loss and terrestrial contamination do not overwhelm the extraterrestrial Ar completely.

Fig. 5. Three-isotope plot of Ne for unmelted AMMs, cosmic spherules, and IDPs. Unmelted AMM data are from Olinger et al. (1990), Osawa and Nagao (2002b), and Osawa et al. (2003). Cosmic spherule data are from Osawa and Nagao (2003) and Osawa et al. (2010). IDP data

**0 0.1 0.2 0.3**

21Ne/22Ne

M240410

IDPs

unmelted micrometeorites cosmic spherules

cosmogenic

Ar isotopic compositions of individual micrometeorites were reported only by two groups, at Washington University and the University of Tokyo (Kehm et al., 1998a; Osawa and Nagao, 2002a, 2002b, 2003; Osawa et al., 2000, 2001, 2003, 2010). Merrihue (1964) reported a low 40Ar/36Ar ratio (172 ± 5 in the 1400°C fraction) in a magnetic separate of Pacific red clay and suggested that it contains meteoritic material, but that the data do not correspond to those of a single micrometeorite. Since Ar has three stable isotopes, as does Ne, the Ar isotopic compositions of micrometeorites can clarify the contributions of more than two components. A three-isotope plot of Ar for unmelted AMMs and cosmic spherules is presented in Fig. 6. IDP data from Kehm et al. (1998a) are not plotted in this diagram because of the lack of raw data. All unmelted micrometeorites with detectable amounts of Ar have 40Ar/36Ar ratios lower than that of the terrestrial atmosphere, 296, confirming their classification as extraterrestrial because terrestrial materials with 40Ar/36Ar ratios lower than that of terrestrial air are very few. Although the Ar isotopic compositions of cosmic spherules have large uncertainties due to the very low Ar concentrations, the 40Ar/36Ar ratios of many spherules are lower than the atmospheric value. This indicates that extraterrestrial Ar is detectable for these samples because significant gas loss and terrestrial contamination do not overwhelm the extraterrestrial Ar

are from Pepin et al. (2000). An arrow shows the direction of cosmogenic Ne.

**3.3 Ar isotopic ratios of micrometeorites** 

**0**

**5**

**10**

20Ne/22Ne

Air

**15**

IFSW

SW

completely.

Fig. 6. Ar isotopic compositions of (a) unmelted AMMs and (b) cosmic spherules. Unmelted AMM data are from Osawa and Nagao (2002b) and Osawa et al. (2003). Cosmic spherule data are from Osawa and Nagao (2003) and Osawa et al. (2010).

Cosmic-ray–produced spallogenic 38Ar was detected only in spherule M240410, which has a detectable amount of cosmogenic 3He and 21Ne. The concentration of cosmogenic 38Ar of the spherule is 4.8 × 10−8 cm3 STP/g, and the CRE age calculated with 2π irradiation is 382.1 Myr (Osawa et al., 2010). All micrometeorites other than this exceptional spherule have no cosmogenic 38Ar, even the unmelted AMMs with relatively high 21Ne/22Ne ratios, presumably due to the lower rate of cosmic-ray production of 38Ar than that of 21Ne (Eugster, 1988).

Fig. 7. 84Kr/20Ne ratios versus 132Xe/20Ne ratios on logarithmic scale. Dotted lines show theoretical fractionation lines of terrestrial air and SW component established by massdependent Rayleigh distillation. A solid line shows a mixing of SW and CM chondrite compositions. Air is from Ozima and Podosek (2002). SW data is represented by the 71501 low-temperature regime in Becker et al. (1989). CM2 chondrite is represented by Belgica-7904 (Nagao et al., 1984). Unmelted AMM data are from Osawa and Nagao (2002b) and Osawa et al. (2003). Cosmic spherule data are from Osawa and Nagao (2003) and Osawa et

In addition, Kr and Xe have no large isotopic anomalies, in contrast with the cases for light noble gases. Indeed the mean values of the Kr and Xe isotopic ratios of micrometeorites are identical within the error to the atmospheric values (Osawa et al., 2000; Osawa and Nagao, 2002a, 2002b). Although micrometeorites may preserve solar-derived Kr and Xe, the isotopic compositions of Kr and Xe are useless to identify the solar component. Even the rocky grains of the asteroid Itokawa recovered by the Hayabusa spacecraft have no Kr and Xe attributable to solar wind, although terrestrial contamination of the samples is very low

The noble gas elemental composition including 84Kr and 132Xe is, however, useful for identifying the sources of heavy noble gases. The relative abundances of 20Ne, 84Kr, and 132Xe are depicted in Fig. 7 on a logarithmic scale. All terrestrial materials are distributed below the theoretical mass fractionation line of SW noble gases because the abundance of

al. (2010).

(Nagao et al., 2011).

The Ar isotopic composition of unmelted AMMs is composed of three components: terrestrial atmosphere, IFSW, and a component of primordial trapped Ar, such as the Q component (Osawa and Nagao, 2002b). Q-noble gas is the main component of heavy noble gases in primitive chondrites hosted by the phase Q, which is an oxidizable phase of a residue of treatment with hydrochloric acid and hydrofluoric acid (e.g., Lewis et al. 1975; Ott et al., 1981; Huss et al., 1996). 38Ar/36Ar ratios that are relatively high compared to the SW value are observed in unmelted AMMs, and the average 38Ar/36Ar ratio of 0.193 is higher than the Q-Ar value of 0.187 (Busemann et al., 2000). This indicates the presence of IFSW Ar, in agreement with the IFSW-like Ne composition shown in Fig. 5. The contribution of unfractionated SW component is small, and fractionated absorbed air need not be considered. In contrast with the cases for He and Ne, the contribution of the primordial trapped Ar component is detectable.

About 40% of cosmic spherules and most unmelted AMMs preserved detectable amounts of extraterrestrial Ar but were affected by atmospheric contamination; most 40Ar in the micrometeorites was dominantly derived from the terrestrial atmosphere. It is not obvious that there exists radiogenic 40Ar produced in situ because 40Ar/36Ar ratios higher than those of the Q or solar components can be explained by atmospheric contamination (Osawa and Nagao, 2002b), and the concentrations of potassium in AMMs are low (Nakamura et al., 1999; Kurat et al., 1994). The enigmatic spherule To440080, however, has an exceptionality high 40Ar/36Ar ratio (566.3 ± 14.8), in spite of the presence of IFSW-like Ne. The high isotopic ratio is clearly due to radiogenic 40Ar. This spherule has a high 36Ar concentration (6.5 × 10−7 cm3 STP/g) in spite of its high 40Ar/36Ar ratio, although meteorites with such high 36Ar concentrations generally have lower 40Ar/36Ar ratios than this spherule. An IFSW 36Ar contribution of approximately 50% is calculated from the concentration of 20Ne, if a 20Ne/36Ar ratio of 47 is adopted as the IFSW ratio (Murer et al., 1997). If this estimation is correct, the original 40Ar/36Ar ratio of this spherule was over 1000, and this spherule undoubtedly originated in a different type of the parent body than did the other micrometeorites.

The contributions of the three Ar components (air, Q, and IFSW) in unmelted AMMs can be estimated using a simple mixing model. In this estimation, all of the 40Ar is assumed to be atmospheric because the 40Ar/36Ar ratios of the IFSW and Q components are inaccurate but assumed to be very low. Atmospheric 36Ar and 38Ar are thus probably overestimated, but they contribute only 5% and 4% of the total Ar, respectively. The contribution of the Q component is comparable to that of IFSW component, and the average contributions of 36Ar and 38Ar of the Q component are found to be 45% and 47% of the total Ar, respectively (Osawa et al., 2002). Since 38Ar/36Ar ratios of cosmic spherules have large uncertainties, as shown in Fig. 6(b), it is difficult to differentiate the contribution of IFSW Ar from that of primordial trapped Ar in individual spherules. The contribution from the Q component may be comparable with that from IFSW component, as it is in the case of unmelted AMMs, since there is no sign that the original noble gas compositions of the cosmic spherules (other than To440080 and M240410) are different from those of the unmelted AMMs. In conclusion, the low 40Ar/36Ar ratios of micrometeorites are not only due to solar wind irradiation.

#### **3.4 Kr and Xe in micrometeorites**

Since Kr and Xe concentrations of single micrometeorites are extremely low, their isotopic compositions cannot be determined accurately, and Kr and Xe isotopic ratios of micrometeorites typically have uncertainties larger than 20% (Osawa and Nagao, 2002b).

The Ar isotopic composition of unmelted AMMs is composed of three components: terrestrial atmosphere, IFSW, and a component of primordial trapped Ar, such as the Q component (Osawa and Nagao, 2002b). Q-noble gas is the main component of heavy noble gases in primitive chondrites hosted by the phase Q, which is an oxidizable phase of a residue of treatment with hydrochloric acid and hydrofluoric acid (e.g., Lewis et al. 1975; Ott et al., 1981; Huss et al., 1996). 38Ar/36Ar ratios that are relatively high compared to the SW value are observed in unmelted AMMs, and the average 38Ar/36Ar ratio of 0.193 is higher than the Q-Ar value of 0.187 (Busemann et al., 2000). This indicates the presence of IFSW Ar, in agreement with the IFSW-like Ne composition shown in Fig. 5. The contribution of unfractionated SW component is small, and fractionated absorbed air need not be considered. In contrast with the cases for He and Ne, the contribution of the primordial trapped Ar component is detectable. About 40% of cosmic spherules and most unmelted AMMs preserved detectable amounts of extraterrestrial Ar but were affected by atmospheric contamination; most 40Ar in the micrometeorites was dominantly derived from the terrestrial atmosphere. It is not obvious that there exists radiogenic 40Ar produced in situ because 40Ar/36Ar ratios higher than those of the Q or solar components can be explained by atmospheric contamination (Osawa and Nagao, 2002b), and the concentrations of potassium in AMMs are low (Nakamura et al., 1999; Kurat et al., 1994). The enigmatic spherule To440080, however, has an exceptionality high 40Ar/36Ar ratio (566.3 ± 14.8), in spite of the presence of IFSW-like Ne. The high isotopic ratio is clearly due to radiogenic 40Ar. This spherule has a high 36Ar concentration (6.5 × 10−7 cm3 STP/g) in spite of its high 40Ar/36Ar ratio, although meteorites with such high 36Ar concentrations generally have lower 40Ar/36Ar ratios than this spherule. An IFSW 36Ar contribution of approximately 50% is calculated from the concentration of 20Ne, if a 20Ne/36Ar ratio of 47 is adopted as the IFSW ratio (Murer et al., 1997). If this estimation is correct, the original 40Ar/36Ar ratio of this spherule was over 1000, and this spherule undoubtedly originated in a

different type of the parent body than did the other micrometeorites.

The contributions of the three Ar components (air, Q, and IFSW) in unmelted AMMs can be estimated using a simple mixing model. In this estimation, all of the 40Ar is assumed to be atmospheric because the 40Ar/36Ar ratios of the IFSW and Q components are inaccurate but assumed to be very low. Atmospheric 36Ar and 38Ar are thus probably overestimated, but they contribute only 5% and 4% of the total Ar, respectively. The contribution of the Q component is comparable to that of IFSW component, and the average contributions of 36Ar and 38Ar of the Q component are found to be 45% and 47% of the total Ar, respectively (Osawa et al., 2002). Since 38Ar/36Ar ratios of cosmic spherules have large uncertainties, as shown in Fig. 6(b), it is difficult to differentiate the contribution of IFSW Ar from that of primordial trapped Ar in individual spherules. The contribution from the Q component may be comparable with that from IFSW component, as it is in the case of unmelted AMMs, since there is no sign that the original noble gas compositions of the cosmic spherules (other than To440080 and M240410) are different from those of the unmelted AMMs. In conclusion, the

low 40Ar/36Ar ratios of micrometeorites are not only due to solar wind irradiation.

Since Kr and Xe concentrations of single micrometeorites are extremely low, their isotopic compositions cannot be determined accurately, and Kr and Xe isotopic ratios of micrometeorites typically have uncertainties larger than 20% (Osawa and Nagao, 2002b).

**3.4 Kr and Xe in micrometeorites** 

Fig. 7. 84Kr/20Ne ratios versus 132Xe/20Ne ratios on logarithmic scale. Dotted lines show theoretical fractionation lines of terrestrial air and SW component established by massdependent Rayleigh distillation. A solid line shows a mixing of SW and CM chondrite compositions. Air is from Ozima and Podosek (2002). SW data is represented by the 71501 low-temperature regime in Becker et al. (1989). CM2 chondrite is represented by Belgica-7904 (Nagao et al., 1984). Unmelted AMM data are from Osawa and Nagao (2002b) and Osawa et al. (2003). Cosmic spherule data are from Osawa and Nagao (2003) and Osawa et al. (2010).

In addition, Kr and Xe have no large isotopic anomalies, in contrast with the cases for light noble gases. Indeed the mean values of the Kr and Xe isotopic ratios of micrometeorites are identical within the error to the atmospheric values (Osawa et al., 2000; Osawa and Nagao, 2002a, 2002b). Although micrometeorites may preserve solar-derived Kr and Xe, the isotopic compositions of Kr and Xe are useless to identify the solar component. Even the rocky grains of the asteroid Itokawa recovered by the Hayabusa spacecraft have no Kr and Xe attributable to solar wind, although terrestrial contamination of the samples is very low (Nagao et al., 2011).

The noble gas elemental composition including 84Kr and 132Xe is, however, useful for identifying the sources of heavy noble gases. The relative abundances of 20Ne, 84Kr, and 132Xe are depicted in Fig. 7 on a logarithmic scale. All terrestrial materials are distributed below the theoretical mass fractionation line of SW noble gases because the abundance of

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terrestrial Xe is low, having been selectively depleted by unknown causes (the so-called "missing Xe"). Extraterrestrial materials can thus be distinguished using the diagram. Most of the unmelted AMM data points do not overlap the area representing terrestrial materials, indicating an extraterrestrial origin of the unmelted AMMs. Most of the unmelted AMMs are distributed above the mass fractionation line of SW noble gases. On the other hand, a few cosmic spherules are plotted in the area representing terrestrial materials, indicating contamination by terrestrial atmosphere.

The solid line shows mixing between SW and the primordial trapped component represented by the noble gas composition of a CM2 chondrite, Belgica-7904 (Nagao et al., 1984)). The noble gas composition of Belgica-7904 mainly reflects the Q component for Kr and Xe and the HL component for Ne. HL gas is a primitive component trapped in presolar diamonds. SW data is substituted for IFSW data in the diagram, under the assumption that there is no difference between IFSW and SW value since the noble gas elemental abundance of IFSW component is unclear. Most unmelted AMMs are distributed between the SW-CM2 chondrite mixing line and the mass fractionation line of SW noble gases. The figure clearly shows that both the primordial trapped component and the SW component are preserved in the micrometeorites. The noble gas compositions of the micrometeorites are thus explained by mixing of three components: a primordial trapped component, SW, and terrestrial contamination. The contribution of each component can be roughly estimated using the simple mixing model. If unfractionated air is assumed in the calculation, the average contributions of atmospheric 84Kr and 132Xe are 1.5% and 2% of the total Kr and Xe, respectively. These values are, however, not accurate because air adsorbed on the surface of micrometeorites should be fractionated and its noble gas elemental ratios cannot be determined accurately (Osawa and Nagao, 2002b). If the elemental compositions of adsorption-fractionated air are arbitrarily set to be 84Kr/20Ne = 0.1 and 132Xe/20Ne = 0.0043, the mean contribution of the fractionated air is only 0.6% of the total 84Kr and 132Xe. 99% of 132Xe and 95% of 84Kr in micrometeorites is due to the primordial trapped component, and the contribution of SW component for Kr and Xe is very low (Osawa et al., 2003). This estimation implies that it is almost impossible to identify the SW Kr and Xe from the isotopic compositions of Kr and Xe.

## **4. Conclusion**

Development of noble gas mass spectrometers has enabled the analysis of single micrometeorites, and noble gas isotopic research has revealed that most micrometeorites collected on the Earth preserved detectable amounts of SW-derived He, Ne, and Ar. However, Kr and Xe are dominated by the primordial component, and solar-derived Xe is almost negligible. The anomalously high 3He/4He ratio and solar-wind–like Ne isotopic composition observed in deep-sea sediments are caused by abundant micrometeorites accumulated on the bottom of the ocean. SW noble gases in micrometeorites were energetically implanted into the surface of micrometeorites in interplanetary space during orbital evolution, but they were partially depleted and fractionated by atmospheric entry heating. Noble gases in cosmic spherules were considerably depleted by harsh heating. The short CRE ages of most micrometeorites inferred from the lack of cosmogenic 21Ne and 38Ar show that the duration of solar wind exposure is less than 1 Myr. Since the terrestrial ages of IDPs and AMMs recovered from fresh Antarctic snow are very low, the trapped SW noble gases in these micrometeorites reflect the composition of recent solar wind.

## **5. References**

136 Exploring the Solar Wind

terrestrial Xe is low, having been selectively depleted by unknown causes (the so-called "missing Xe"). Extraterrestrial materials can thus be distinguished using the diagram. Most of the unmelted AMM data points do not overlap the area representing terrestrial materials, indicating an extraterrestrial origin of the unmelted AMMs. Most of the unmelted AMMs are distributed above the mass fractionation line of SW noble gases. On the other hand, a few cosmic spherules are plotted in the area representing terrestrial materials, indicating

The solid line shows mixing between SW and the primordial trapped component represented by the noble gas composition of a CM2 chondrite, Belgica-7904 (Nagao et al., 1984)). The noble gas composition of Belgica-7904 mainly reflects the Q component for Kr and Xe and the HL component for Ne. HL gas is a primitive component trapped in presolar diamonds. SW data is substituted for IFSW data in the diagram, under the assumption that there is no difference between IFSW and SW value since the noble gas elemental abundance of IFSW component is unclear. Most unmelted AMMs are distributed between the SW-CM2 chondrite mixing line and the mass fractionation line of SW noble gases. The figure clearly shows that both the primordial trapped component and the SW component are preserved in the micrometeorites. The noble gas compositions of the micrometeorites are thus explained by mixing of three components: a primordial trapped component, SW, and terrestrial contamination. The contribution of each component can be roughly estimated using the simple mixing model. If unfractionated air is assumed in the calculation, the average contributions of atmospheric 84Kr and 132Xe are 1.5% and 2% of the total Kr and Xe, respectively. These values are, however, not accurate because air adsorbed on the surface of micrometeorites should be fractionated and its noble gas elemental ratios cannot be determined accurately (Osawa and Nagao, 2002b). If the elemental compositions of adsorption-fractionated air are arbitrarily set to be 84Kr/20Ne = 0.1 and 132Xe/20Ne = 0.0043, the mean contribution of the fractionated air is only 0.6% of the total 84Kr and 132Xe. 99% of 132Xe and 95% of 84Kr in micrometeorites is due to the primordial trapped component, and the contribution of SW component for Kr and Xe is very low (Osawa et al., 2003). This estimation implies that it is almost impossible to identify the SW Kr and Xe from the

Development of noble gas mass spectrometers has enabled the analysis of single micrometeorites, and noble gas isotopic research has revealed that most micrometeorites collected on the Earth preserved detectable amounts of SW-derived He, Ne, and Ar. However, Kr and Xe are dominated by the primordial component, and solar-derived Xe is almost negligible. The anomalously high 3He/4He ratio and solar-wind–like Ne isotopic composition observed in deep-sea sediments are caused by abundant micrometeorites accumulated on the bottom of the ocean. SW noble gases in micrometeorites were energetically implanted into the surface of micrometeorites in interplanetary space during orbital evolution, but they were partially depleted and fractionated by atmospheric entry heating. Noble gases in cosmic spherules were considerably depleted by harsh heating. The short CRE ages of most micrometeorites inferred from the lack of cosmogenic 21Ne and 38Ar show that the duration of solar wind exposure is less than 1 Myr. Since the terrestrial ages of IDPs and AMMs recovered from fresh Antarctic snow are very low, the trapped SW noble

gases in these micrometeorites reflect the composition of recent solar wind.

contamination by terrestrial atmosphere.

isotopic compositions of Kr and Xe.

**4. Conclusion** 


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**Part 3** 

**The Solar Wind Dynamics – From Large to Small Scales** 

